Security key derivation in dual connectivity

ABSTRACT

The invention relates to methods for establishing a secure communication link between a mobile station and a secondary base station in a mobile communication system. The invention is also providing mobile communication system for performing these methods, and computer readable media the instructions of which cause the mobile communication system to perform the methods described herein. Specifically, the invention suggests that in response to the detected or signaled potential security breach, the master base station increments a freshness counter for re-initializing the communication between the mobile station and the secondary base station; and the mobile station and the secondary base station re-initialize the communication there between. The re-initialization is performed under the control of the master base station and further includes deriving a same security key based on said incremented freshness counter, and establishing the secure communication link utilizing the same, derived security key.

FIELD OF THE INVENTION

The invention relates to methods for establishing a secure communicationlink between a mobile station and a secondary base station in a mobilecommunication system comprising the mobile station, a master and thesecondary base station. The invention is also providing a mobile stationand base stations for participating and for performing the methodsdescribed herein.

TECHNICAL BACKGROUND Long Term Evolution (LTE)

Third-generation mobile systems (3G) based on WCDMA radio-accesstechnology are being deployed on a broad scale all around the world. Afirst step in enhancing or evolving this technology entails introducingHigh-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, alsoreferred to as High Speed Uplink Packet Access (HSUPA), giving a radioaccess technology that is highly competitive.

In order to be prepared for further increasing user demands and to becompetitive against new radio access technologies, 3GPP introduced a newmobile communication system which is called Long Term Evolution (LTE).LTE is designed to meet the carrier needs for high speed data and mediatransport as well as high capacity voice support for the next decade.The ability to provide high bit rates is a key measure for LTE.

The work item (WI) specification on Long-Term Evolution (LTE) calledEvolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial RadioAccess Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTEsystem represents efficient packet-based radio access and radio accessnetworks that provide full IP-based functionalities with low latency andlow cost. In LTE, scalable multiple transmission bandwidths arespecified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order toachieve flexible system deployment using a given spectrum. In thedownlink, Orthogonal Frequency Division Multiplexing (OFDM) based radioaccess was adopted because of its inherent immunity to multipathinterference (MPI) due to a low symbol rate, the use of a cyclic prefix(CP) and its affinity to different transmission bandwidth arrangements.Single-carrier frequency division multiple access (SC-FDMA) based radioaccess was adopted in the uplink, since provisioning of wide areacoverage was prioritized over improvement in the peak data rateconsidering the restricted transmit power of the user equipment (UE).Many key packet radio access techniques are employed includingmultiple-input multiple-output (MIMO) channel transmission techniquesand a highly efficient control signaling structure is achieved in LTERel. 8/9.

LTE Architecture

The overall architecture is shown in FIG. 1 and a more detailedrepresentation of the E-UTRAN architecture is given in FIG. 2. TheE-UTRAN consists of an eNodeB, providing the E-UTRA user plane(PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towardsthe user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY),Medium Access Control (MAC), Radio Link Control (RLC) and Packet DataControl Protocol (PDCP) layers that include the functionality ofuser-plane header-compression and encryption. It also offers RadioResource Control (RRC) functionality corresponding to the control plane.It performs many functions including radio resource management,admission control, scheduling, enforcement of negotiated uplink Qualityof Service (QoS), cell information broadcast, ciphering/deciphering ofuser and control plane data, and compression/decompression ofdownlink/uplink user plane packet headers. The eNodeBs areinterconnected with each other by means of the X2 interface.

The eNodeBs are also connected by means of the S1 interface to the EPC(Evolved Packet Core), more specifically to the MME (Mobility ManagementEntity) by means of the S1-MME and to the Serving Gateway (SGW) by meansof the S1-U. The S1 interface supports a many-to-many relation betweenMMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user datapackets, while also acting as the mobility anchor for the user planeduring inter-eNodeB handovers and as the anchor for mobility between LTEand other 3GPP technologies (terminating S4 interface and relaying thetraffic between 2G/3G systems and PDN GW). For idle state userequipment, the SGW terminates the downlink data path and triggers pagingwhen downlink data arrives for the user equipment. It manages and storesuser equipment contexts, e.g. parameters of the IP bearer service,network internal routing information. It also performs replication ofthe user traffic in case of lawful interception.

The MME is the key control-node for the LTE access-network. It isresponsible for idle mode user equipment tracking and paging procedureincluding retransmissions. It is involved in the beareractivation/deactivation process and is also responsible for choosing theSGW for a user equipment at the initial attach and at time of intra-LTEhandover involving Core Network (CN) node relocation. It is responsiblefor authenticating the user (by interacting with the HSS). TheNon-Access Stratum (NAS) signaling terminates at the MME and it is alsoresponsible for generation and allocation of temporary identities touser equipment. It checks the authorization of the user equipment tocamp on the service provider's Public Land Mobile Network (PLMN) andenforces user equipment roaming restrictions. The MME is the terminationpoint in the network for ciphering/integrity protection for NASsignaling and handles the security key management. Lawful interceptionof signaling is also supported by the MME. The MME also provides thecontrol plane function for mobility between LTE and 2G/3G accessnetworks with the S3 interface terminating at the MME from the SGSN. TheMME also terminates the S6a interface towards the home HSS for roaminguser equipment.

Component Carrier Structure in LTE

The downlink component carrier of a 3GPP LTE system is subdivided in thetime-frequency domain in so-called sub-frames. In 3GPP LTE eachsub-frame is divided into two downlink slots as shown in FIG. 3, whereinthe first downlink slot comprises the control channel region (PDCCHregion) within the first OFDM symbols. Each sub-frame consists of a givenumber of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPPLTE (Release 8)), wherein each OFDM symbol spans over the entirebandwidth of the component carrier. The OFDM symbols thus each consistsof a number of modulation symbols transmitted on respective N_(RB)^(DL)×N_(sc) ^(RB) subcarriers as also shown in FIG. 4.

Assuming a multi-carrier communication system, e.g. employing OFDM, asfor example used in 3GPP Long Term Evolution (LTE), the smallest unit ofresources that can be assigned by the scheduler is one “resource block”.A physical resource block (PRB) is defined as N_(symb) ^(DL) consecutiveOFDM symbols in the time domain (e.g. 7 OFDM symbols) and N_(sc) ^(RB)subcarriers in the frequency domain as exemplified in FIG. 4 (e.g. 12subcarriers for a component carrier). In 3GPP LTE (Release 8), aphysical resource block thus consists of N_(symb) ^(DL)×N_(sc) ^(RB)resource elements, corresponding to one slot in the time domain and 180kHz in the frequency domain (for further details on the downlinkresource grid, see for example 3GPP TS 36.211, “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation(Release 8)”, section 6.2, available at http://www.3gpp.org andincorporated herein by reference).

One sub-frame consists of two slots, so that there are 14 OFDM symbolsin a sub-frame when a so-called “normal” CP (cyclic prefix) is used, and12 OFDM symbols in a sub-frame when a so-called “extended” CP is used.For sake of terminology, in the following the time-frequency resourcesequivalent to the same N_(sc) ^(RB) subcarriers spanning a fullsub-frame is called a “resource block pair”, or equivalent “RB pair” or“PRB pair”.

The term “component carrier” refers to a combination of several resourceblocks in the frequency domain. In future releases of LTE, the term“component carrier” is no longer used; instead, the terminology ischanged to “cell”, which refers to a combination of downlink andoptionally uplink resources. The linking between the carrier frequencyof the downlink resources and the carrier frequency of the uplinkresources is indicated in the system information transmitted on thedownlink resources.

Similar assumptions for the component carrier structure apply to laterreleases too.

Carrier Aggregation in LTE-A for Support of Wider Bandwidth

The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07). Although the overall frequencyspectrum for IMT-Advanced was decided, the actual available frequencybandwidth is different according to each region or country. Followingthe decision on the available frequency spectrum outline, however,standardization of a radio interface started in the 3rd GenerationPartnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the StudyItem description on “Further Advancements for E-UTRA (LTE-Advanced)” wasapproved. The study item covers technology components to be consideredfor the evolution of E-UTRA, e.g. to fulfill the requirements onIMT-Advanced.

The bandwidth that the LTE-Advanced system is able to support is 100MHz, while an LTE system can only support 20 MHz. Nowadays, the lack ofradio spectrum has become a bottleneck of the development of wirelessnetworks, and as a result it is difficult to find a spectrum band whichis wide enough for the LTE-Advanced system. Consequently, it is urgentto find a way to gain a wider radio spectrum band, wherein a possibleanswer is the carrier aggregation functionality.

In carrier aggregation, two or more component carriers (componentcarriers) are aggregated in order to support wider transmissionbandwidths up to 100 MHz. Several cells in the LTE system are aggregatedinto one wider channel in the LTE-Advanced system which is wide enoughfor 100 MHz even though these cells in LTE are in different frequencybands.

All component carriers can be configured to be LTE Rel. 8/9 compatible,at least when the aggregated numbers of component carriers in the uplinkand the downlink are the same. Not all component carriers aggregated bya user equipment may necessarily be Rel. 8/9 compatible. Existingmechanism (e.g. barring) may be used to avoid Rel-8/9 user equipment tocamp on a component carrier.

A user equipment may simultaneously receive or transmit one or multiplecomponent carriers (corresponding to multiple serving cells) dependingon its capabilities. A LTE-A Rel. 10 user equipment with receptionand/or transmission capabilities for carrier aggregation cansimultaneously receive and/or transmit on multiple serving cells,whereas an LTE Rel. 8/9 user equipment can receive and transmit on asingle serving cell only, provided that the structure of the componentcarrier follows the Rel. 8/9 specifications.

Carrier aggregation is supported for both contiguous and non-contiguouscomponent carriers with each component carrier limited to a maximum of110 Resource Blocks in the frequency domain using the 3GPP LTE (Release8/9) numerology.

It is possible to configure a 3GPP LTE-A (Release 10) compatible userequipment to aggregate a different number of component carriersoriginating from the same eNodeB (base station) and of possiblydifferent bandwidths in the uplink and the downlink. The number ofdownlink component carriers that can be configured depends on thedownlink aggregation capability of the UE. Conversely, the number ofuplink component carriers that can be configured depends on the uplinkaggregation capability of the UE. It may not be possible to configure amobile terminal with more uplink component carriers than downlinkcomponent carriers.

In a typical TDD deployment, the number of component carriers and thebandwidth of each component carrier in uplink and downlink is the same.Component carriers originating from the same eNodeB need not to providethe same coverage.

The spacing between center frequencies of contiguously aggregatedcomponent carriers shall be a multiple of 300 kHz. This is in order tobe compatible with the 100 kHz frequency raster of 3GPP LTE (Release8/9) and at the same time preserve orthogonality of the subcarriers with15 kHz spacing. Depending on the aggregation scenario, the n×300 kHzspacing can be facilitated by insertion of a low number of unusedsubcarriers between contiguous component carriers.

The nature of the aggregation of multiple carriers is only exposed up tothe MAC layer. For both uplink and downlink there is one HARQ entityrequired in MAC for each aggregated component carrier. There is (in theabsence of SU-MIMO for uplink) at most one transport block per componentcarrier. A transport block and its potential HARQ retransmissions needto be mapped on the same component carrier.

The Layer 2 structure with activated carrier aggregation is shown inFIG. 5 and FIG. 6 for the downlink and uplink respectively.

When carrier aggregation is configured, the mobile terminal only has oneRRC connection with the network. At RRC connectionestablishment/re-establishment, one cell provides the security input(one ECGI, one PCI and one ARFCN) and the non-access stratum mobilityinformation (e.g. TAI) similarly as in LTE Rel. 8/9. After RRCconnection establishment/re-establishment, the component carriercorresponding to that cell is referred to as the downlink Primary Cell(PCell). There is always one and only one downlink PCell (DL PCell) andone uplink PCell (UL PCell) configured per user equipment in connectedstate. Within the configured set of component carriers, other cells arereferred to as Secondary Cells (SCells); with carriers of the SCellbeing the Downlink Secondary Component Carrier (DL SCC) and UplinkSecondary Component Carrier (UL SCC). The characteristics of thedownlink and uplink PCell are:

For each SCell the usage of uplink resources by the UE, in addition tothe downlink ones is configurable; the number of DL SCCs configured istherefore always larger or equal to the number of UL SCCs, and no SCellcan be configured for usage of uplink resources only

The uplink PCell is used for transmission of Layer 1 uplink controlinformation

The downlink PCell cannot be de-activated, unlike SCells

From UE perspective, each uplink resource only belongs to one servingcell

The number of serving cells that can be configured depends on theaggregation capability of the UE

Re-establishment is triggered when the downlink PCell experiencesRayleigh fading (RLF), not when downlink SCells experience RLF

The downlink PCell cell can change with handover (i.e. with security keychange and RACH procedure)

Non-access stratum information is taken from the downlink PCell

PCell can only be changed with handover procedure (i.e. with securitykey change and RACH procedure)

PCell is used for transmission of PUCCH

The configuration and reconfiguration of component carriers can beperformed by RRC. Activation and deactivation is done via MAC controlelements. At intra-LTE handover, RRC can also add, remove, orreconfigure SCells for usage in the target cell. When adding a newSCell, dedicated RRC signaling is used for sending the systeminformation of the SCell, the information being necessary fortransmission/reception (similarly as in Rel-8/9 for handover).

When a user equipment is configured with carrier aggregation there isone pair of uplink and downlink component carriers that is alwaysactive. The downlink component carrier of that pair might be alsoreferred to as ‘DL anchor carrier’. Same applies also for the uplink.

When carrier aggregation is configured, a user equipment may bescheduled over multiple component carriers simultaneously but at mostone random access procedure shall be ongoing at any time. Cross-carrierscheduling allows the PDCCH of a component carrier to schedule resourceson another component carrier. For this purpose a component carrieridentification field is introduced in the respective DCI formats, calledCIF.

A linking between uplink and downlink component carriers allowsidentifying the uplink component carrier for which the grant applieswhen there is no cross-carrier scheduling. The linkage of downlinkcomponent carriers to uplink component carrier does not necessarily needto be one to one. In other words, more than one downlink componentcarrier can link to the same uplink component carrier. At the same time,a downlink component carrier can only link to one uplink componentcarrier.

Small Cell Deployment Scenarios

Explosive demands for mobile data are driving changes in how mobileoperators will need to respond to the challenging requirements of highercapacity and improved Quality of user Experience (QoE). Currently,fourth generation wireless access systems using Long Term Evolution(LTE) are being deployed by many operators worldwide in order to offerfaster access with lower latency and more efficiency than 3G/3.5Gsystem.

The anticipated future traffic growth is so tremendous that there is avastly increased need for further network densification to handle thecapacity requirements, particularly in high traffic areas (hot spotareas) that generate the highest volume of traffic. Networkdensification—increasing the number of network nodes, thereby bringingthem physically closer to the user terminals—is a key to improvingtraffic capacity and extending the achievable user-data rates of awireless communication system.

In addition to straightforward densification of a macro deployment,network densification can be achieved by the deployment of complementarylow-power nodes respectively small cells under the coverage of anexisting macro-node layer. In such a heterogeneous deployment, thelow-power nodes provide very high traffic capacity and very high userthroughput locally, for example in indoor and outdoor hotspot locations.Meanwhile, the macro layer ensures service availability and QoE over theentire coverage area. In other words, the layer containing the low-powernodes can also be referred to as providing local-area access, incontrast to the wide-area-covering macro layer.

The installation of low-power nodes respectively small cells as well asheterogeneous deployments has been possible since the first release ofLTE. In this regard, a number of solutions have been specified in recentreleases of LTE (i.e., Release-10/11). More specifically, these recentreleases introduced additional tools to handle inter-layer interferencein heterogeneous deployments. In order to further optimize performanceand provide cost/energy-efficient operation, small cells require furtherenhancements and in many cases need to interact with or complementexisting macro cells.

Such optimizations are to be investigated as part of the furtherevolution of LTE—Release 12 and beyond. In particular furtherenhancements related to low-power nodes and heterogeneous deploymentswill be considered under the umbrella of the new Rel-12 study item (SI)“Study on Small Cell Enhancements for E-UTRA and E-UTRAN”. Some of theseactivities will focus on achieving an even higher degree of interworkingbetween the macro and low-power layers, including different forms ofmacro assistance to the low-power layer and dual-layer connectivity.Dual connectivity implies that the device has simultaneous connectionsto both macro and low-power layers.

Dual Connectivity

One promising solution to the problems which are currently underdiscussion in 3GPP RAN working groups is the so-called dual connectivityconcept. Dual connectivity is used to refer to an operation where agiven UE consumes radio resources provided by at least two differentnetwork nodes connected via a non-ideal backhaul.

In other words, in dual connectivity the UE is connected with both amacro cell (master or macro eNB) and small cell (secondary or smalleNB). Furthermore, each eNB involved in dual connectivity for a UE mayassume different roles. Those roles do not necessarily depend on theeNB's power class and can vary among UEs.

For use of a consistent terminology, reference is made to Stage 2description (3GPP R2-140906) of Small Cell Enhancement in LTE where thefollowing terms are defined as follows. A Master Cell Group, MCG, indual connectivity describes a group of serving cells associated with theMeNB, comprising of the PCell and optionally one or more SCells. Themaster eNB in dual connectivity identifies the eNB which terminates atleast S1-MME. In this respect, the term MCG bearer in dual connectivityrefers to radio protocols only located in the MeNB to use MeNBresources.

Similarly, a Secondary Cell Group, SCG in dual connectivity describes agroup of serving cells associated with the SeNB comprising of thespecial SCell and optionally one or more SCells. The secondary eNB indual connectivity identifies the eNB that is providing additional radioresources for the UE but is not the Master eNB. In this respect, theterm SCG bearer in dual connectivity refers to radio protocols onlylocated in the secondary eNB to use secondary eNB resources.

Since the study item is currently at a very early stage, details on thedeployment of dual connectivity are yet to be decided. For example,different architectures are still actively discussed and, hence, mayinfluence implementation aspects of dual connectivity. Therefore, manyissues/details, e.g. protocol enhancements, are still open for furtherdevelopment.

FIG. 7 shows an exemplary architecture for dual connectivity.Specifically, an architecture is illustrated which corresponds to whatis currently understood as Architecture 1A. In this Architecture 1A,S1-U terminates in the master eNB and in the secondary eNB and theS1-MME is terminated in the master eNB.

Both the master eNB and the secondary eNB provide independently thefunctionality of the Packet Data Convergence Protocol (PDCP) such thatthe illustrated Architecture 1A is not necessary to provide splitbearers, i.e. where the a bearer is split over the master eNB and thesecondary eNB.

In general, it should be understood the depicted dual connectivityarchitecture 1A is only one among many options for realizing dualconnectivity. Moreover, the concept of dual connectivity applies thefollowing assumptions on the architecture:

Per bearer level decision where to serve each packet, C/U plane split

As an example UE RRC signaling and high QoS data such as VoLTE can beserved by the Macro cell, while best effort data is offloaded to thesmall cell.

No coupling between bearers, so no common PDCP or RLC required betweenthe Macro cell and small cell

Looser coordination between RAN nodes SeNB has no connection to S-GW,i.e. packets are forwarded by MeNB

Small Cell is transparent to CN.

Regarding the last two bullet points, it should be noted that it's alsopossible that SeNB is connected directly with the S-GW, i.e. S1-U isbetween S-GW and SeNB. Essentially, there are three different optionsw.r.t. the bearer mapping/splitting:

-   -   Option 1: S1-U also terminates in SeNB, as; depicted in FIG. 7;    -   Option 2: S1-U terminates in MeNB, no bearer split in RAN; and    -   Option 3: S1-U terminates in MeNB, bearer split in RAN.

Security

Security is a very important feature of 3GPP LTE and in 3GPP TS 33.401:“3rd Generation Partnership Project; Technical Specification GroupServices and System Aspects; 3GPP System Architecture Evolution (SAE);Security architecture (Release 12)”, Version 12.10.0, section 4,available at http://www.3gpp.org and incorporated herein by reference,defines five security feature groups. Each of these feature groups meetscertain threats and accomplishes certain security objectives:

-   -   Network access security (I) relates to the set of security        features that provide users with secure access to services, and        which in particular protect against attacks on the (radio)        access link.    -   Network domain security (II) relates to the set of security        features that enable nodes to securely exchange signaling data,        user data (between AN and SN and within AN), and protect against        attacks on the wireline network.    -   User domain security (III) relates to the set of security        features that secure access to mobile stations.    -   Application domain security (IV) relates to the set of security        features that enable applications in the user and in the        provider domain to securely exchange messages.    -   Visibility and configurability of security (V) relates to the        set of features that enables the user to inform himself whether        a security feature is in operation or not and whether the use        and provision of services should depend on the security feature.

The security objectives are illustrated in FIG. 8 with regard to theinteraction between units and between functional layers in LTE. In theremaining document, the discussion focuses on network access security.

User Data (and Signaling Data) Confidentiality: Ciphering

The user data (and signaling data) must be ciphered. The User planeconfidentiality protection shall be done at PDCP layer and is anoperator option. The user plane data is ciphered by the PDCP protocolbetween the UE and the eNB as specified in 3GPP TS 36.323: “3rdGeneration Partnership Project; Technical Specification Group RadioAccess Network; Evolved Universal Terrestrial Radio Access (E-UTRA);Packet Data Convergence Protocol (PDCP) specification (Release 11)”,Version 11.2.0, section 5.6, available at http://www.3gpp.org andincorporated herein by reference.

Requirements for Handling User Plane Date for the eNB

It is eNB's task to cipher and decipher user plane packets between theUu reference point and the S1/X2 reference points and to handleintegrity protection for user plane packets for the S1/X2 referencepoints.

1. User plane data ciphering/deciphering and integrity handling shalltake place inside the secure environment where the related keys arestored.

2. The transport of user data over S1-U and X2-U shall be integrity,confidentially and replay-protected from unauthorized parties. If thisis to be accomplished by cryptographic means, clause 12 shall be appliedexcept for the Un interface between RN and DeNB.

Requirements for Handling Control Plane Date for the eNB

It is eNB's task to provide confidentiality and integrity protection forcontrol plane packets on the S1/X2 reference points.

1. Control plane data ciphering/deciphering and integrity handling shalltake place inside the secure environment where the related keys arestored.

2. The transport of control plane data over S1-MME and X2-C shall beintegrity-, confidentiality- and replay-protected from unauthorizedparties. If this is to be accomplished by cryptographic means, clause 11shall be applied except for the Un interface between RN and DeNB.

EPS Key Hierarchy

Requirements on EPC and E-UTRAN related to keys:

a) The EPC and E-UTRAN shall allow for use of encryption and integrityprotection algorithms for AS and NAS protection having keys of length128 bits and for future use the network interfaces shall be prepared tosupport 256 bit keys.

b) The keys used for UP, NAS and AS protection shall be dependent on thealgorithm with which they are used.

The key hierarchy is shown in FIG. 9 includes following keys: K_(eNB),K_(NAsint), K_(NAsenc), K_(UPenc), K_(RRCint) and K_(RRCenc). In thefollowing, reference is made to a Key Derivation Function, KDF, which isspecified in Annex A.7 of 3GPP TS 33.401: “3rd Generation PartnershipProject; Technical Specification Group Services and System Aspects; 3GPPSystem Architecture Evolution (SAE); Security architecture (Release12)”, Version 12.10.0, section 4, available at http://www.3gpp.org andincorporated herein by reference.

K_(eNB) is a key derived by ME and MME from K_(ASME) or by ME and targeteNB.

Keys for NAS traffic:

K_(NASint) is a key, which shall only be used for the protection of NAStraffic with a particular integrity algorithm This key is derived by MEand MME from K_(ASME), as well as an identifier for the integrityalgorithm using the KDF.

K_(NAsenc) is a key, which shall only be used for the protection of NAStraffic with a particular encryption algorithm. This key is derived byME and MME from K_(ASME), as well as an identifier for the encryptionalgorithm using the KDF.

Keys for UP traffic:

K_(UPenc) is a key, which shall only be used for the protection of UPtraffic with a particular encryption algorithm. This key is derived byME and eNB from K_(eNB), as well as an identifier for the encryptionalgorithm using the KDF.

K_(UPint) is a key, which shall only be used for the protection of UPtraffic between RN and DeNB with a particular integrity algorithm. Thiskey is derived by RN and DeNB from K_(eNB), as well as an identifier forthe integrity algorithm using the KDF.

Keys for RRC traffic:

K_(RRCint) is a key, which shall only be used for the protection of RRCtraffic with a particular integrity algorithm. K_(RRCint) is derived byME and eNB from K_(eNB), as well as an identifier for the integrityalgorithm using the KDF.

K_(RRCenc) is a key, which shall only be used for the protection of RRCtraffic with a particular encryption algorithm. K_(RRCenc) is derived byME and eNB from K_(eNB) as well as an identifier for the encryptionalgorithm using the KDF.

Intermediate keys:

NH, referring to the Next Hop parameter, is a key derived by ME and MMEto provide forward security.

K_(eNB)* is a key derived by ME and eNB when performing a horizontal orvertical key derivation.

Specifically, the key handling in handover is described in section 7.2.8of 3GPP TS 33.401: “3rd Generation Partnership Project; TechnicalSpecification Group Services and System Aspects; 3GPP SystemArchitecture Evolution (SAE); Security architecture (Release 12)”,Version 12.10.0, available at http://www.3gpp.org and incorporatedherein by reference.

For Dual Connectivity, S-K_(eNB) will be derived from the K_(eNB) and a“freshness parameter” which will be for example be 16 bit long.

Intra-eNB Handover

When the eNB decides to perform an intra-eNB handover it shall deriveK_(eNB)* as in Annex A.5 using target PCI, its frequency EARFCN-DL, andeither NH or the current K_(eNB) depending on the following criteria:the eNB shall use the NH for deriving K_(eNB)* if an unused {NH, NCC}pair is available in the eNB (this is referred to as a vertical keyderivation), otherwise if no unused {NH, NCC} pair is available in theeNB, the eNB shall derive K_(eNB)* from the current K_(eNB) (this isreferred to as a horizontal key derivation).

The eNB shall use the K_(eNB)* as the K_(eNB) after handover. The eNBshall send the NCC used for K_(eNB)* derivation to UE in HO Commandmessage.

X2-Hanover

As in intra-eNB handovers, for X2 handovers the source eNB shall performa vertical key derivation in case it has an unused {NH, NCC} pair. Thesource eNB shall first compute K_(eNB)* from target PCI, its frequencyEARFCN-DL, and either from currently active K_(eNB) in case ofhorizontal key derivation or from the NH in case of vertical keyderivation as described in Annex A.5 of 3GPP TS 33.401: “3rd GenerationPartnership Project; Technical Specification Group Services and SystemAspects; 3GPP System Architecture Evolution (SAE); Security architecture(Release 12)”, Version 12.10.0, available at http://www.3gpp.org andincorporated herein by reference.

Next the source eNB shall forward the {K_(eNB)*, NCC} pair to the targeteNB. The target eNB shall use the received K_(eNB)* directly as K_(eNB)to be used with the UE. The target eNB shall associate the NCC valuereceived from source eNB with the K_(eNB). The target eNB shall includethe received NCC into the prepared HO Command message, which is sentback to the source eNB in a transparent container and forwarded to theUE by source eNB.

When the target eNB has completed the handover signaling with the UE, itshall send a S1 PATH SWITCH REQUEST to the MME. Upon reception of the S1PATH SWITCH REQUEST, the MME shall increase its locally kept NCC valueby one and compute a new fresh NH by using the K_(ASME) and its locallykept NH value as input to the function defined in Annex A.4. The MMEshall then send the newly computed {NH, NCC} pair to the target eNB inthe S1 PATH SWITCH REQUEST ACKNOWLEDGE message. The target eNB shallstore the received {NH, NCC} pair for further handovers and remove otherexisting unused stored {NH, NCC} pairs if any.

KeNB Refresh

This procedure is based on an intra-cell handover. The K_(eNB) chainingthat is performed during a handover ensures that the K_(eNB) isrefreshed with respect to the RRC and UP COUNT after the procedure.

128-Bit Ciphering Algorithm Inputs and Outputs

The input parameters to the ciphering algorithm are a 128-bit cipher keynamed KEY, a 32-bit COUNT, a 5-bit bearer identity BEARER, the 1-bitdirection of the transmission i.e. DIRECTION, and the length of thekeystream required i.e. LENGTH. The DIRECTION bit shall be 0 for uplinkand 1 for downlink.

FIG. 10 illustrates the use of the ciphering algorithm EEA to encryptplaintext by applying a keystream using a bit per bit binary addition ofthe plaintext and the keystream. The plaintext may be recovered bygenerating the same keystream using the same input parameters andapplying a bit per bit binary addition with the ciphertext.

The use and mode of operation of the 128-EEA algorithms are specified inAnnex B of 3GPP TS 33.401: “3rd Generation Partnership Project;Technical Specification Group Services and System Aspects; 3GPP SystemArchitecture Evolution (SAE); Security architecture (Release 12)”,Version 12.10.0, available at http://www.3gpp.org and incorporatedherein by reference.

The input parameters to the 128-bit EEA algorithms are an 128-bit cipherkey K_(UPenc) as KEY, a 5-bit bearer identity BEARER which value isassigned as specified by PDCP, the 1-bit direction of transmissionDIRECTION, the length of the keystream required LENGTH and a bearerspecific, time and direction dependent 32-bit input COUNT whichcorresponds to the 32-bit PDCP COUNT.

Based on the input parameters the algorithm generates the outputkeystream block KEYSTREAM which is used to encrypt the input plaintextblock PLAINTEXT to produce the output ciphertext block CIPHERTEXT. Theinput parameter LENGTH shall affect only the length of the KEYSTREAMBLOCK, not the actual bits in it.

Shortcomings of Prior Art Power Control

In dual connectivity, the security key S-K_(eNB) is applicable tonetwork access security for the secondary base station, SeNB and iscomparable in its functionality to security key K_(eNB) for the masterbase station, MeNB. The derivation of this security key S-K_(eNB) shallbe explained in connection with FIG. 12.

The master base station, MeNB, derives, in step 3, this security keyS-K_(eNB) for the communication between the mobile terminal and thesecondary base station from the security key K_(eNB) and a “Counter”value. Subsequently, the derived S-K_(eNB) is transmitted by the masterbase station, MeNB, in step 4, to the secondary base station, SeNB.Thereafter, the secondary base station, SeNB derives, in step 5, theUP-encryption keys S-K_(UPenc), and optionally integrity protection keyS-K_(UPint), from this S-K_(eNB). Specifically, the SeNB derives furtherthe UP-integrity keys from this S-K_(eNB) (and if required the integritykeys for RRC Signaling and User plane and also the RRC-encryption keys).The Counter is a new 16 bit (or perhaps a different length) counter andis known as “freshness parameter”.

As seen from FIG. 10, for the ciphering/encryption apart from KEY(KeNB/S-KeNB), four other input parameters are prescribed for theciphering operation of data. Security is based on the principle that allfive input parameters all should not be the same for subsequentciphering. If they are, it represents a potential security threat. Inputparameters COUNT, DIRECTION and LENGTH do not allow much degree offreedom for eNB to choose/change between different values; e.g. For ULdata ciphering, the DIRECTION has to indicate “UL”. For DL dataciphering, the DIRECTION has to indicate “DL”.

A problem results from situations when the bearer identification (i.e.RB-id) is to be reused as the BEARER input parameter to theciphering/encryption (e.g. a same RB-id is allocated to a new bearer). Asimilar problem results from situations where COUNT would wrap-up (for asame BEARER). In these situations, if the KEY (KeNB/S-KeNB) would remainthe same, then it would lead to a repetition in the input parameters.Such a repetition in the input parameters to the ciphering/encryptionrepresents a security hole that can be exploited. For example, RB-idreuse security hole can be exploited by an attacker quickly adding moreand more application.

For ciphering and integrity a COUNT value is maintained which is shownin FIG. 11. The COUNT value is composed of a HFN and the PDCP sequencenumber, SN. The length of the PDCP SN is configured by upper layers. ThePDCP SN is bearer specific which means that for each radio bearer, aseparate PDCP SN is maintained. The size of the HFN part in bits isequal to 32 minus the length of the PDCP SN. COUNT wrap-up would happenwhen the number of PDCP PDUs transmitted exceeds the total length ofCOUNT.

Bearer (RB-id) reuse (same RB-id being allocated to a new bearer) couldhappen in the following situations: Firstly, a bearer release and latersame RB-id (especially a DRB) being allocated to a different bearer. ADRB-id is the RB-id allocated to a Data Bearer (DRB). Secondly, whenDRB-id space is wrapped i.e. 29 DRBs in LTE (32−3 SRBs) were establishedand a new Bearer will need to use one of the already used DRB-id. Atotal of 32 Bearers can be configured to a UE in LTE out of which 3bearers (and corresponding Ids) are reserved for Signaling, calledSignaling Radio Bearer.

Since dual connectivity has not yet been standardized, the abovediscussed problems are new and there are no solutions available inspecification. However, based on the principles in legacy (e.g. LTE Rel.11 and before) an eNB avoids RB-id reuse as much as possible. However,there may be a point beyond which the situation (RB-id reuse or COUNTwrap-up) cannot be avoided anymore.

When avoiding is no more possible/is difficult, the MeNB may performIntra-cell handover which changes the K_(eNB) to be used in the MeNB(MCG) but further it is not clear how it will lead to refreshing ofS-K_(eNB). When/if the new K_(eNB) is used to refresh the S-K_(eNB), theMeNB bearers also get interrupted unnecessarily since the refresh ofK_(eNB) was only a means of refreshing S-K_(eNB) but was not requiredotherwise (no RB-id reuse or COUNT wrap-up for the bearers being servedby MeNB directly). Further, Intra-cell handover is quite expensiveprocedure since it involves some interruption of user data not only inSeNB but also in MeNB. Interruption of user data for MeNB only bearersis quite un-necessary/avoidable since this is mainly a security issue atthe SeNB.

As a result of Intra-cell handover all the bearer(s) needs to bere-established and data forwarded between the network nodes, etc. Sothis is better avoided or optimized.

Dual Connectivity introduces more than one eNB to UE's connection i.e.the UE consumes/utilizes resources from two eNBs. Both security protectthe signaling and/or data towards the UE with their respective keys;MeNB with K_(eNB) and SeNB with S-K_(eNB).

SUMMARY OF THE INVENTION

One object of the invention is to provide an improved method forestablishing a secure communication link between a master base stationand a secondary base station in case of a detected security breach,thereby avoiding the problems of the prior art as identified above.

The object is solved by the subject matter of the independent claims.Advantageous embodiments are subject to the dependent claims.

It is assumed that the mobile station is in dual connectivity and thusconnected to both a master base station and a secondary base station viarespective communication links. As explained above, in dual connectivitythe security key S-K_(eNB) for the Secondary Cell Group, SCG, (i.e. forcommunication with the secondary base station) is inter-dependent on thesecurity key K_(eNB) for the Master Cell Group, MCG (i.e. forcommunication with the master base station).

In this respect, in case of the detection of a potential securitybreach, the network would trigger an intra-cell handover for all bearersof the master and the secondary cell group thereby re-establishingsecurity for the communication with the master and the secondary basestation.

According to a first aspect of the invention, in case of the detectionof a potential security breach it is proposed to only re-establishsecurity for the communication with the secondary base station and notwith the master base station. For an independent re-establishment ofsecurity for the communication with the secondary base station, a newsecurity key S-K_(eNB) is derived on the basis of a incremented, hence,new COUNT input parameter to the ciphering/encryption algorithm(henceforth: freshness counter). In other words, the security keyK_(eNB) for the communication with the master base station can remainthe same, such that no intra-cell handover becomes necessary.

Specifically, in response to a detected potential security breach, themaster base station increments the freshness counter for re-initializingthe communication between the mobile station and the secondary basestation. The term re-initializing of communication shall, in the contextof the invention, be understood as re-establishing the PDCP sub-layer,re-establishing the RLC sub-layer and resetting the MAC layer.

In this respect, the re-initializing of communication differs from theexecution of a handover command in that it provides for an optimizedrouting of packet data units, namely since the C-RNTI does not change.More importantly, the re-initialization of communication is triggered bya different message, namely an RRC connection reconfiguration messagewhich does not include a handover command (i.e. without mobility controlinformation).

The re-initialization of the communication between the mobile stationand the secondary base station is based on the incremented freshnesscounter in that a new security key S-K_(eNB) is derived for thecommunication there between under the control of the master basestation. In this respect, the mobile station and the secondary basestation are capable of establishing the secure communication link, afterdetection of the potential security breach, using the same, derivedsecurity key S-K_(eNB).

As the above procedure is so different from what is presently carriedout upon detection of a potential security breach, independently fromthe above, the following suggested improving the re-establishment ofsecurity within a secondary cell group.

According to a second aspect of the invention, a mobile station (i.e.UE) is suggested, the mobile station being initialized for communicationwith a master and a secondary base station in a wireless communicationsystem, wherein the mobile station receives from the master base stationa reconfiguration message (for example an RRC connection reconfigurationmessage) which includes a freshness counter.

For clarification purposes only, it shall be emphasized that thereconfiguration message does not include a handover command.

In response to receipt of the reconfiguration message, the mobilestation derives a security key S-K_(eNB) for the communication with thesecondary base station. More particularly, the mobile station derivesthe security key S-K_(eNB) based on the incremented freshness counterincluded in the re-configuration message.

The secondary key S-K_(eNB), is subsequently used, by the mobilestation, for re-initializing the communication with the secondary basestation, thereby enabling the mobile station to establish a securecommunication link with the secondary base station.

Notably, the re-configuration message received from the master basestation includes the incremented freshness counter. Accordingly, themobile station can derive that it is to trigger re-initialization of thecommunication with the secondary base station. In other words, shouldthe re-configuration message not be for the communication with thesecondary base station no incremented freshness counter would beincluded therein.

Advantageously, in case the mobile station receives the re-configurationmessage in a ciphered form from the master base station, the mobilestation may determine if it is also is provided with a current versionof the security key K_(eNB) on which, in addition to the incrementedfreshness counter, the derived security key for the communication withthe secondary base station is based.

According to a first embodiment, in line with the first aspect of theinvention, a method is proposed for establishing a secure communicationlink between a mobile station and a secondary base station in a mobilecommunication system. The mobile communication system comprises themobile station, a master and the secondary base station. The mobilestation is initialized for communication with the master and thesecondary base station.

The master or the secondary base station detect a potential securitybreach including a condition where a sequence counter of packet dataunits of an established secure communication link between the mobilestation and the secondary base station is to wrap-up sinceinitialization of the communication between the mobile station and thesecondary base station, and a condition where a communication linkidentification is to be reused for establishing the secure communicationlink with the secondary base station since initialization of thecommunication between the mobile station and the secondary base station.

In case the potential security breach is detected by the secondary basestation, the secondary base station signals the detected security breachto the master base station. In response to the detected or signaledpotential security breach, the master base station increments afreshness counter for re-initializing the communication between themobile station and the secondary base station; and the mobile stationand the secondary base station, re-initialize the communication therebetween. The re-initialization is performed under the control of themaster base station and further includes deriving a same security keybased on said incremented freshness counter, and establishing the securecommunication link utilizing the same, derived security key.

According to a second embodiment, in line with the first aspect of theinvention, a mobile communication system is suggested for establishing asecure communication link between a mobile station and a secondary basestation. The mobile communication system comprises the mobile station, amaster and the secondary base station. The mobile station is initializedfor communication with the master and the secondary base station.

The master and/or the secondary base station are configured to detect apotential security breach including: a condition where a sequencecounter of packet data units of an established secure communication linkbetween the mobile station and the secondary base station is to wrap-upsince initialization of the communication between the mobile station andthe secondary base station, and a condition where a communication linkidentification is to be reused for establishing the secure communicationlink with the secondary base station since initialization of thecommunication between the mobile station and the secondary base station.

The secondary base station is configured to, in case of detecting thepotential security breach, signal the detected security breach to themaster base station; the master base station is configured to incrementin response to the detected or signaled potential security breach, afreshness counter for re-initializing the communication between themobile station and the secondary base station.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention is described in more detail withreference to the attached figures and drawings.

FIG. 1 shows an exemplary architecture of a 3GPP LTE system,

FIG. 2 shows an exemplary overview of the overall E-UTRAN architectureof 3GPP LTE,

FIG. 3 shows exemplary sub-frame boundaries on a downlink componentcarrier as defined for 3GPP LTE (Release 8/9),

FIG. 4 shows an exemplary downlink resource grid of a downlink slot asdefined for 3GPP LTE (Release 8/9),

FIGS. 5 & 6 show the Layer 2 structure in 3GPP LTE (Release 10) withactivated carrier aggregation for the downlink and uplink, respectively,

FIG. 7 details the architecture of a mobile communication system in dualconnectivity with macro and small eNBs connected to the core network in3GPP LTE (Release 11),

FIG. 8 gives an overview of the security architecture in 3GPP LTE(Release 12),

FIG. 9 details the security key architecture in 3GPP LTE (Release 12),

FIG. 10 illustrates the ciphering/encryption algorithm in 3GPP LTE(Release 12),

FIG. 11 shows the format of COUNT being an input parameter to theciphering/encryption algorithm in 3GPP LTE (Release 11),

FIG. 12 illustrates the security key derivation in a wirelesscommunication system in dual connectivity in 3GPP LTE (Release 12), and

FIGS. 13-16 illustrates the security key derivation according to variousembodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A mobile station or mobile node is a physical entity within acommunication network. One node may have several functional entities. Afunctional entity refers to a software or hardware module thatimplements and/or offers a predetermined set of functions to otherfunctional entities of a node or the network. Nodes may have one or moreinterfaces that attach the node to a communication facility or mediumover which nodes can communicate. Similarly, a network entity may have alogical interface attaching the functional entity to a communicationfacility or medium over it may communicate with other functionalentities or correspondent nodes.

The term “master base station” used in the claims and throughout thedescription of the invention is to be construed as used in the field ofdual connectivity of 3GPP LTE-A; thus, other terms are macro basestation, or master/macro eNB; or serving base station or any otherterminology to be decided later by 3GPP. Similarly, the term “secondarybase station” used in the claims and throughout the description is to beconstrued as used in the field of dual connectivity of 3GPP LTE-A; thus,other terms are slave base station, or secondary/slave eNB or any otherterminology to be decided later by 3GPP.

The term “radio link” or “communication link” used in the claims andthroughout the description of the invention is to be understood in abroad way as the radio connection between the mobile station and a basestation including the master base station or the secondary base station.

Further, the term of “initializing” or “re-initializing” used in theclaims and throughout the description of the invention is to beunderstood as including the (re-)establishment of the Radio LinkControl, RLC, sub-layer, the (re-)establishment of the Packet DataConvergence Protocol, PDCP sub-layer, and the setup (reset) of theMedium Access Control, MAC, layer, respectively. In this respect,service data units, SDUs, and/or packet data units, PDUs, which were notsuccessfully transmitted prior to the re-initialization will betransmitted in compliance with re-initialized communication therebyenforcing network access security.

In the following, several embodiments of the invention will be explainedin detail. For exemplary purposes only, most of the embodiments areoutlined in relation to a radio access scheme according to 3GPP LTE(Release 8/9) and LTE-A (Release 10/11) mobile communication systems,partly discussed in the Technical Background section above. It should benoted that the invention may be advantageously used for example in amobile communication system such as 3GPP LTE-A (Release 12)communication systems as described in the Technical Background sectionabove. These embodiments are described as implementations for use inconnection with and/or for enhancement of functionality specified in3GPP LTE and/or LTE-A. In this respect, the terminology of 3GPP LTEand/or LTE-A is employed throughout the description. Further, exemplaryconfigurations are explored to detail the full breadth of the invention.

The explanations should not be understood as limiting the invention, butas a mere example of the invention's embodiments to better understandthe invention. A skilled person should be aware that the generalprinciples of the invention as laid out in the claims can be applied todifferent scenarios and in ways that are not explicitly describedherein. Correspondingly, the following scenarios assumed for explanatorypurposes of the various embodiments shall not limit the invention assuch.

The invention will be described with reference to FIGS. 13 to 16. In thefollowing, a dual connectivity scenario in a small cell environment isassumed for the wireless communication system. In this respect, themobile station is connected to both a master base station and asecondary base station respectively via a first and secondarycommunication link. It should be noted however that the invention is notrestricted to this scenario; for instance, scenarios where the mobilestation is connected to a master base station and at least two secondarybase stations are also possible.

In more detail, the mobile station is initialized for communication withthe master base station and the secondary base station. In this respect,the mobile station is configured with a RLC sub-layer and a PDCPsub-layer for communication with the respective master base station andthe secondary base station. In view of security aspects, theinitialization for communication also means that the mobile station isprovided with security keys which allow the mobile station to establishsecure communication links to the master and the secondary base station.

Specifically, the initialization of the mobile station prescribes that asecurity key K_(eNB) is provided to, more specifically derived by, themobile station for establishing a secure communication link with themaster base station. From an implementation point of view, the securitykey K_(eNB) may be used by the mobile station to derive further securitykeys for specific purposes, namely for ciphering or for integrity. Inthis context, the technical background section lists the K_(UPint), theK_(UPenc), the K_(RRCint), and the K_(RRCenc). Irrespective thereof, themobile station is enabled to enforce security upon a communication linkwith the master base station.

Similarly, the initialization of the mobile station prescribes that alsoa security key S-K_(eNB) is provided to, more specifically derived by,the mobile station for the communication with the secondary basestation. Specifically, this security key S-K_(eNB) is derived from thesecurity key K_(eNB) for the communication with the master base stationand an additional parameter named freshness counter. In other words, onthe basis of the security key K_(eNB) and the freshness counter, themobile station is capable of deriving security key S-K_(eNB) therebyallowed to enforce security upon a communication link with the secondarybase station.

With respect to the communication between the mobile station and themaster base station, a potential security breach may be detected. Amongvarious conditions, it becomes readily apparent that a potentialsecurity breach results from situations where the ciphering/encryptionalgorithm is provided with repeating input parameters.

In the context of the invention, a potential security breach includes acondition where a sequence counter of packet data units, PDUs, of anestablished secure communication link is about to wrap-up (i.e. exceedsa predefined threshold related to the maximum number of the sequencecounter) or actually wraps-up (i.e. exceeds the maximum number of thesequence counter) since initialization of the communication between themobile station and the secondary base station. In other words, in caseof the sequence counter relating to an established secure communicationlink wrap-ups, same communication link is no longer secure because thesequence number(s) has(have) to be reused.

More particularly, the sequence counter of PDUs, on the basis of whichthe COUNT parameter is formed by the PDCP sequence number and the HyperFrame Number, HFN as shown in FIG. 11, is an input to theciphering/encryption algorithm. Accordingly, a repetition of thesequence number of PDUs would result in a potential security breach.Consequently, such a potential security breach is detectable by way ofdetecting a condition where the sequence counter of PDUs wraps-up sinceinitialization.

In the context of the invention, a potential security breach alsoincludes the condition where the communication link identification, moreparticularly, the resource bearer identification RB-id, is to be reused(i.e. previously unused communication link identifications are no longeravailable) since initialization of the communication between the mobilestation and the secondary base station. The reuse of a communicationlink identification may result from establishing a new securecommunication link. In other words, in case the plurality of availablecommunication link identifications is already used up, establishment ofa further communication link is not secure since it involves reusing acommunication link identification.

More particularly, the communication link identification (i.e. RB-id) inform of the parameter BEARER is a further input to theciphering/encryption algorithm such that repetition thereof would alsoresult in a potential security breach. Consequently, such a potentialsecurity breach is detectable by way of detecting a condition where thecommunication link identification is to be reused since initialization.

In more general, the above discussed potential security breach relate toan initialized communication between the mobile station and thesecondary base station only. In this respect, should the communicationbetween the mobile station and the secondary base station bere-initialized, the detection of conditions which result in a potentialsecurity breach start anew. Consequently, the conditions of a sequencecounter wrap-up or of a communication link identification reuse are onlydecisive if they occur since initialization of the communication betweenthe mobile station and the secondary base station.

According to a more particular embodiment, the condition where thesequence counter of PDUs is about to wrap-up is detected by thesecondary base station. Specifically, this secondary base station isprovided for integrity reasons with a sequence counter for PDUs as partof the communication with the mobile station. Consequently, thesecondary base station may detect, since initial of the communicationwith the mobile station, the sequence counter is about to wrap-up.

In another more particular embodiment, of the condition where thecommunication link identification is about to be reused may either bedetected by the master base station or the secondary base station. Sincethe dual connectivity scenario requires both, the master and thesecondary base station to be informed of the establishment of a newsecure communication link between the mobile station and the secondarybase station, both base stations may monitor the communication linkidentification assignment and, specifically, the reuse thereof sinceinitialization.

Should a potential security breach be detected by the secondary basestation, same secondary base station signals the detected securitybreach to the master base station. Alternatively, should a potentialsecurity breach be detected by the master base station, no signaling tothe master base station is required.

In response to the signaled or detected potential security breach, themaster base station averts same potential security breach by triggersre-initialization of the communication between the mobile station andthe secondary base station. In particular, for this purpose, the masterbase station increments the freshness counter on which the security keyS-K_(eNB) for the communication between the mobile station and the basestation is based.

Then, the mobile station and the secondary base station re-initializethe communication between each other under the control of the masterbase station, namely by the mobile station and the secondary basestation, deriving a same security key S-K_(eNB) on the basis of theincremented freshness counter. In particular, the master base stationcontrols derivation of the same security key S-K_(eNB) by providing theincremented freshness counter to the mobile station and to the secondarybase station.

Consequently, having re-initialized the communication the mobile stationand secondary base station are capable of establishing a securecommunication link between each other utilizing the same, derivedsecurity key S-K_(eNB).

Advantageously, in this embodiment the communication between the mobilestation and the secondary base station re-initialized without havingre-initialized the communication between the mobile station and themaster base station. In other words, for the communication between themobile station and the secondary base station a same secure keyS-K_(eNB) is derived which bases on a incremented, hence different,freshness counter and thereby enables establishing a securecommunication link there between without requiring re-initialization ofthe communication with the master base station, inherently requiringderivation of a corresponding security key K_(eNB).

First Example of a More Detailed Embodiment of the Invention

Referring now to FIG. 13 wherein a first example of a more detailedembodiment of the invention is shown. This embodiment illustrates amethod for establishing a secure communication link between a mobilestation and a secondary base station in a mobile communication systememploying dual connectivity. Accordingly, the mobile communicationsystem comprises a master and a secondary base station. The mobilestation is initialized for communicating with the master base stationand the secondary base station.

Supposing that the communication between the mobile station and themaster and the secondary base station is respectively initialized,either the master or the secondary base station perform, in step 1, theoperation of detecting a potential security breach. As discussed above,the potential security breach may be detected as a condition where asequence counter of PDUs is about to wrap-up or where a communicationlink identification is to be reused since initialization of thecommunication between the mobile station and the secondary base station.

In case the secondary base station detects, for example, the conditionwhere a sequence counter of packet data units of an established securecommunication link between the mobile station and the secondary basestation is about to wrap-up or actually wraps-up, same secondary basestation signals, in step 2, the detected security breach to the masterbase station. Since in an alternative case, the master base station mayequally detect a security breach, the signaling of the detected securitybreach is indicated as being optional by way of a dashed line.

Signaling of the detected security breach by the secondary base stationto the master base station may correspond to a message requesting thechange of the security key S-K_(eNB) on which the communication betweenmobile station and the secondary base station is based.

In response to the detected or signaled security breach, the master basestation increments, in step 3, a freshness counter maintained for thecommunication between the mobile station and the secondary base station.This freshness counter is for re-initializing the communication betweenthe mobile station and the secondary base station in that it allowsenforcing network access security there between.

Subsequently, the master base station, in step 4, derives on the basisof the incremented freshness counter a new security key S-K_(eNB) forthe communication between the mobile station and the secondary basestation. As discussed in the description above, the derivation of thesecurity key S-K_(eNB) is not only based on the incremented freshnesscounter but also on the security key K_(eNB) for the communicationbetween the mobile station and the master base station which is alsoavailable to both communication partners.

Advantageously, the present embodiment dispenses with the need for themaster base station to re-initialize this communication between themobile station and the master base station, and, hence, with the needfor the master base station to derive a new security key K_(eNB)therefore.

Having derived the new security key S-K_(eNB) for the communicationbetween the mobile station and the secondary base station, the masterbase station transmits, in step 5, this newly derived security keyS-K_(eNB) and the incremented freshness to the secondary base station.From an implementation side, the transmission of the newly derivedsecurity key S-K_(eNB) and the incremented freshness counter may beeffected utilizing the X2 interface.

Subsequently, the secondary base station generates, in step 6, are-configuration message including the incremented freshness counter forre-initializing the communication between the mobile station and thesecondary base station. The secondary base station then transmits samemessage to the master base station. From an implementation side, thetransmission of the re-configuration message may be effected utilizingthe X2 interface.

In an implementation of the present embodiment, the re-configurationmessage is an RRC connection reconfiguration message and includesadditional information commonly included in the message. In particular,the RRC connection reconfiguration message additionally includes adedicated Random Access CHannel, RACH, preamble. For illustrativepurposes, the dedicated RACH preamble is titled RACH preamble-Y.

The re-configuration message, generated by the secondary base station,is then forwarded, in step 7, by the master base station to the mobilestation. Even though the re-configuration message is received from themaster base station, the mobile station can identify from its contentthat it is generated by and, hence, is linked to (i.e. related to) thecommunication with the secondary base station and not to thecommunication with the master base station.

The skilled reader can readily appreciate from the discussion above,that the re-configuration message forwarded by the master base stationto the mobile station is ciphered by the master base station using thesecurity key K_(eNB) for communication between each other. In thisrespect, security can be enforced for the transmission of theincremented freshness counter without adding implementation complexity,simply due to the fact that that the present embodiment dispenses withthe need for re-initializing the communication between the mobilestation and the master base station.

In the context of the invention, the reception of this re-configurationmessage by the mobile station may be understood as a trigger for themobile station to perform re-initialization of the communication withthe secondary base station.

Accordingly, the mobile station derives, in step 8, a security keyS-K_(eNB) for the communication with the secondary base station on thebasis of the freshness counter included in the forwardedre-configuration message. This security key S-K_(eNB), which is derivedby the mobile station, is same to the security key S-K_(eNB) derived instep 4 by the master base station and transmitted in step 5 to thesecondary base station.

Consequently, since both, the mobile station and the secondary basestation are provided with the same, derived security key S-K_(eNB), asecure communication link can be established on the basis of thissecurity key S-K_(eNB) between the mobile station and the secondary basestation.

In a variation of step 8, the mobile station first determines whetherthe incremented freshness counter, included in the transmittedre-configuration message, differs from a previous freshness counterincluded in a previously transmitted re-configuration message, and onlyin case of a different, incremented freshness counter, the mobilestation derives the same security key S-K_(eNB).

Subsequently, the mobile station re-initializes, in step 9, thecommunication with the secondary base station on the basis of thederived, same security key (cf. step 8) for the communication betweenthe mobile station and the secondary base station. From theimplementation side, the re-initialization performed by the mobilestation may include re-establishing the PDCP sub-layer, re-establishingthe RLC sub-layer and resetting the MAC layer.

Further to the implementation of the present embodiment where there-configuration message, being a RRC connection reconfigurationmessage, includes dedicated RACH preamble-Y, this dedicated RACHpreamble-Y is subsequently utilized for performing, in step 10, a RACHprocedure between the mobile station and the secondary base station.

Advantageously, the RACH procedure between the mobile station and thesecondary base station not only updates the timing advance forcommunication there between but also may be understood, in the contextof the invention, as a trigger for the secondary base station to performre-initialization of the communication with the mobile station.

In this respect, the secondary base station re-initializes, in step 11,the communication with the mobile station on the basis of the derived,same security key (cf. step 4 and 5) for the communication between themobile station and the secondary base station. From the implementationside, the re-initialization performed by the mobile station may includere-establishing the PDCP sub-layer, re-establishing the RLC sub-layerand resetting the MAC layer.

Subsequently, the mobile station transmits, in step 12, are-configuration complete message to the master base station which, instep 13, is forwarded by the master base station to the secondary basestation. Particularly, in the present embodiment, the re-configurationcomplete message is transmitted by the mobile station after the mobilestation and the secondary base station have both re-initialized thecommunication there between.

This may be achieved, for example, by the mobile station deferring thetransmission of the re-configuration complete message to the master basestation by a predetermined time interval. Alternatively, the mobilestation may also assume that the secondary base station re-initializesin step 10 its connection faster than the transmission and theforwarding of the re-configuration complete message in steps 12 and 13and may immediately transmit same message after completion of the RACHprocedure in step 10.

The skilled reader can readily appreciate from the discussion above,that the re-configuration complete message, forwarded by the mobilestation to the master base station, is ciphered by the mobile stationusing the security key K_(eNB) for communication between each other. Inthis respect, security can be enforced also for the transmission of thisre-configuration complete message. From an implementation side, thetransmission of the re-configuration complete message between the basestations in step 13 may be effected via the X2 interface.

Even further to the implementation of the present embodiment, inresponse to the re-configuration message, being a RRC connectionreconfiguration message, the re-configuration complete message is an RRCconnection reconfiguration complete message.

Consequently, having re-initialized the communication in steps 9 and 11,the mobile station and secondary base station are capable ofestablishing a secure communication link between each other utilizingthe same, derived security key S-K_(eNB).

Advantageously, also in the present embodiment the communication betweenthe mobile station and the secondary base station re-initialized withouthaving to re-initialize the communication between the mobile station andthe master base station. In other words, for the communication betweenthe mobile station and the secondary base station a same secure keyS-K_(eNB) is derived which bases on a incremented, hence different,freshness counter and thereby enables establishing a securecommunication link there between without requiring re-initialization ofthe communication with the master base station, inherently requiringderivation of a corresponding security key K_(eNB).

Second Example of a More Detailed Embodiment of the Invention

Referring now to FIG. 14 wherein a second example of a more detailedembodiment of the invention is shown. This embodiment also illustrates amethod for establishing a secure communication link between a mobilestation and a secondary base station in a mobile communication systememploying dual connectivity. Accordingly, the mobile communicationsystem comprises a master and a secondary base station. The mobilestation is initialized for communicating with the master base stationand the secondary base station.

In contrast to the first example of a more detailed embodiment, in thepresent embodiment the re-configuration message including the freshnesscounter is generated and transmitted directly by the master base stationto the mobile station and does not require generating and transmittingsame re-configuration message by the secondary base station andsubsequently forwarding it by the master base station to the mobilestation. Accordingly, the present embodiment results in an overallquicker secure communication link establishment between the mobilestation and the secondary base station because the re-configurationmessage is only transmitted once instead of twice.

Supposing that the communication between the mobile station and themaster and the secondary base station is respectively initialized,either the master or the secondary base station perform, in step 1, theoperation of detecting a potential security breach. As discussed above,the potential security breach may be detected as a condition where asequence counter of PDUs is about to wrap-up or where a communicationlink identification is to be reused since initialization of thecommunication between the mobile station and the secondary base station.

In case the secondary base station detects, for example, the conditionwhere a sequence counter of packet data units of an established securecommunication link between the mobile station and the secondary basestation is about to wrap-up or actually wraps-up, same secondary basestation signals, in step 2, the detected security breach to the masterbase station. Since in an alternative case, the master base station mayequally detect a security breach, the signaling of the detected securitybreach is indicated as being optional by way of a dashed line.

Signaling of the detected security breach by the secondary base stationto the master base station may correspond to a message requesting thechange of the security key S-K_(eNB) on which the communication betweenmobile station and the secondary base station is based.

In response to the detected or signaled security breach, the master basestation increments, in step 3, a freshness counter maintained for thecommunication between the mobile station and the secondary base station.This freshness counter is for re-initializing the communication betweenthe mobile station and the secondary base station in that it allowsenforcing network access security there between.

Subsequently, the master base station, in step 4, derives on the basisof the incremented freshness counter a new security key S-K_(eNB) forthe communication between the mobile station and the secondary basestation. As discussed in the description above, the derivation of thesecurity key S-K_(eNB) is not only based on the incremented freshnesscounter but also on the security key K_(eNB) for the communicationbetween the mobile station and the master base station which is alsoavailable to both communication partners.

Advantageously, the present embodiment dispenses with the need for themaster base station to re-initialize this communication between themobile station and the master base station, and, hence, with the needfor the master base station to derive a new security key K_(eNB)therefore.

Having derived the new security key S-K_(eNB) for the communicationbetween the mobile station and the secondary base station, the masterbase station generates, in step 5, a re-configuration message includingthe incremented freshness counter for re-initializing the communicationbetween the mobile station and the secondary base station. The masterbase station then transmits same message to the mobile station.

In the present embodiment, the mobile station, receiving there-configuration message including the incremented freshness counter,link this message to the communication with the secondary base stationdue to the mere fact that it includes the freshness counter. Are-configuration message for re-configuring the communication with themaster base station does not included a freshness counter. In thisrespect, the mobile station can identify from the message's content thatit is linked to (i.e. related to) the communication with the secondarybase station and not to the communication with the master base station.

In an implementation of the present embodiment, the re-configurationmessage is an RRC connection reconfiguration message and includesadditional information commonly included in the message. In particular,the RRC connection reconfiguration message additionally includes adedicated Random Access CHannel, RACH, preamble. For illustrativepurposes, the dedicated RACH preamble is titled RACH preamble-Y.

Subsequently, the master base station transmits, in step 6, the newlyderived security key S-K_(eNB) and the incremented freshness counter tothe secondary base station. From an implementation side, thetransmission of the newly derived security key S-K_(eNB) and theincremented freshness counter may be effected utilizing the X2interface.

The skilled reader can readily appreciate from the discussion above,that the re-configuration message transmitted by the master base stationto the mobile station is ciphered by the master base station using thesecurity key K_(eNB) for communication between each other. In thisrespect, security can be enforced for the transmission of theincremented freshness counter without adding implementation complexity,simply due to the fact that that the present embodiment dispenses withthe need for re-initializing the communication between the mobilestation and the master base station.

Furthermore, from the description above, it becomes apparent that thesteps 5 and 6 may also be carried out by the master base station inreversed order, namely that the re-configuration message including theincremented freshness counter is transmitted (cf. step 5) to the mobilestation after, and not before transmitting (cf. step 6) the newlyderived security key S-K_(eNB) and the incremented freshness counter tothe secondary base station.

In the context of the invention, the reception of this re-configurationmessage by the mobile station may be understood as a trigger for themobile station to perform re-initialization of the communication withthe secondary base station.

Accordingly, the mobile station derives, in step 7, a security keyS-K_(eNB) for the communication with the secondary base station on thebasis of the freshness counter included in the forwardedre-configuration message. This security key S-K_(eNB), which is derivedby the mobile station, is same to the security key S-K_(eNB) derived instep 4 by the master base station and transmitted in step 6 to thesecondary base station.

Consequently, since both, the mobile station and the secondary basestation are provided with the same, derived security key S-K_(eNB), asecure communication link can be established on the basis of thissecurity key S-K_(eNB) between the mobile station and the secondary basestation.

In a variation of step 7, the mobile station first determines whetherthe incremented freshness counter, included in the transmittedre-configuration message, differs from a previous freshness counterincluded in a previously transmitted re-configuration message, and onlyin case of a different, incremented freshness counter, the mobilestation derives the same security key S-K_(eNB).

Subsequently, the mobile station re-initializes, in step 8, thecommunication with the secondary base station on the basis of thederived, same security key (cf. step 8) for the communication betweenthe mobile station and the secondary base station. From theimplementation side, the re-initialization performed by the mobilestation may include re-establishing the PDCP sub-layer, re-establishingthe RLC sub-layer and resetting the MAC layer.

Further to the implementation of the present embodiment where there-configuration message, being a RRC connection reconfigurationmessage, includes dedicated RACH preamble-Y, this dedicated RACHpreamble-Y is subsequently utilized for performing, in step 9, a RACHprocedure between the mobile station and the secondary base station.

Advantageously, the RACH procedure between the mobile station and thesecondary base station not only updates the timing advance forcommunication there between but also may be understood, in the contextof the invention, as a trigger for the secondary base station to performre-initialization of the communication with the mobile station.

In this respect, the secondary base station re-initializes, in step 10,the communication with the mobile station on the basis of the derived,same security key (cf. steps 4, 6 and 7) for the communication betweenthe mobile station and the secondary base station. From theimplementation side, the re-initialization performed by the mobilestation may include re-establishing the PDCP sub-layer, re-establishingthe RLC sub-layer and resetting the MAC layer.

Subsequently, the mobile station transmits, in step 11, are-configuration complete message to the master base station which, instep 12, is forwarded by the master base station to the secondary basestation. Particularly, in the present embodiment, the re-configurationcomplete message is transmitted by the mobile station after the mobilestation and the secondary base station have both re-initialized thecommunication there between.

This may be achieved, for example, by the mobile station deferring thetransmission of the re-configuration complete message to the master basestation by a predetermined time interval. Alternatively, the mobilestation may also assume that the secondary base station re-initializesin step 10 its connection faster than the transmission and theforwarding of the re-configuration complete message in steps 11 and 12and may immediately transmit same message after completion of the RACHprocedure in step 9.

The skilled reader can readily appreciate from the discussion above,that the re-configuration complete message, forwarded by the mobilestation to the master base station, is ciphered by the mobile stationusing the security key K_(eNB) for communication between each other. Inthis respect, security can be enforced also for the transmission of thisre-configuration complete message. From an implementation side, thetransmission of the re-configuration complete message between the basestations in step 13 may be effected utilizing the X2 interface.

Even further to the implementation of the present embodiment, inresponse to the re-configuration message, being a RRC connectionreconfiguration message, the re-configuration complete message is an RRCconnection reconfiguration complete message.

Consequently, having re-initialized the communication in steps 8 and 10,the mobile station and secondary base station are capable ofestablishing a secure communication link between each other utilizingthe same, derived security key S-K_(eNB).

Advantageously, also in the present embodiment the communication betweenthe mobile station and the secondary base station re-initialized withouthaving to re-initialize the communication between the mobile station andthe master base station. In other words, for the communication betweenthe mobile station and the secondary base station a same secure keyS-K_(eNB) is derived which bases on a incremented, hence different,freshness counter and thereby enables establishing a securecommunication link there between without requiring re-initialization ofthe communication with the master base station, inherently requiringderivation of a corresponding security key K_(eNB).

In an alternative implementation of the present embodiment, there-configuration message, being a RRC connection reconfigurationmessage, may include a dedicated preamble from among a plurality ofpre-configured preambles, or instead, may include information forindicating a dedicated preamble from among the plurality ofpre-configured preambles, namely for performing a RACH procedure betweenthe mobile station and the secondary base station. In thisimplementation, the master base station controls pre-configuration ofthe plurality of pre-configured preambles at the secondary base station,for example, using the X2 interface.

More particularly, this implementation assumes a scenario where, for thecommunication between the mobile station and the secondary base station,a plurality of RACH preambles is pre-configured. Specifically,pre-configuration of RACH preambles may be considered advantageous insituations where not only one but a plurality of mobile stations want toestablish a secure communication link with this secondary base station.

Having preemptively pre-configured the secondary base station with theplurality of preambles for performing the RACH procedure between themobile station and same secondary base station, the mobile station maybe transmitted/indicated a dedicated preamble from the plurality ofpreambles to the mobile station, i.e. without the need for requestingthe secondary base station to assign a specific dedicated preamble forthe RACH procedure to be performed. In other words, by pre-configuringthe secondary base station with the plurality of preambles, thisplurality is reserved for the specific purpose of the master basestation transmitting/indicating from among the plurality the dedicatedpreamble to be used for performing a RACH procedure, this specificpurpose prevents the secondary base station from assigning itdifferently as a dedicated preamble.

Consequently, pre-configuring the second base station with the pluralityof preambles from among which the master base station cantransmit/indicate a dedicated preamble to be used for performing theRACH procedure dispenses with the need for coordinating which of thededicated preambles of the secondary base station is to betransmitted/indicated by the master base station to the mobile terminal.

Moreover, a distinction between transmitting and indicating of adedicated preamble in the RRC connection reconfiguration message by themaster base station to the mobile station may be useful in case of asmall number of preambles (e.g. less than 16 preambles represented with4 bits) which are pre-configured for performing the RACH procedurebetween the mobile station and the master base station. In such a case,the transmission efficiency for the RRC connection reconfigurationmessage improves since less signaling bits are required.

Third Example of a More Detailed Embodiment of the Invention

Referring now to FIG. 15 wherein a third example of a more detailedembodiment of the invention is shown. This embodiment also illustrates amethod for establishing a secure communication link between a mobilestation and a secondary base station in a mobile communication systememploying dual connectivity. Accordingly, the mobile communicationsystem comprises a master and a secondary base station. The mobilestation is initialized for communicating with the master base stationand the secondary base station.

In contrast to the second example of a more detailed embodiment, in thepresent embodiment a scenario is assumed where no dedicated preamble isor can be assigned for performing the RACH procedure between the mobilestation and the secondary base station. Nonetheless, also in thisembodiment a trigger is required for the secondary base station tosynchronize performing re-initialization of the communication with themobile station.

In this respect, the present embodiment proposes transmitting, by themobile station, a re-configuration acknowledge message to the secondarybase station (cf. step 9). Thereby, the implementation complexity in thepresent embodiment can be reduced, namely by omitting the assignment ofa dedicated preamble to the mobile station for performing the RACHprocedure between it and the secondary base station. Further, in thepresent embodiment the processing time of establishing the securecommunication link between the mobile station and the secondary basestation reduces.

Supposing that the communication between the mobile station and themaster and the secondary base station is respectively initialized,either the master or the secondary base station perform, in step 1, theoperation of detecting a potential security breach. As discussed above,the potential security breach may be detected as a condition where asequence counter of PDUs is about to wrap-up or where a communicationlink identification is to be reused since initialization of thecommunication between the mobile station and the secondary base station.

In case the secondary base station detects, for example, the conditionwhere a sequence counter of packet data units of an established securecommunication link between the mobile station and the secondary basestation is about to wrap-up or actually wraps-up, same secondary basestation signals, in step 2, the detected security breach to the masterbase station. Since in an alternative case, the master base station mayequally detect a security breach, the signaling of the detected securitybreach is indicated as being optional by way of a dashed line.

Signaling of the detected security breach by the secondary base stationto the master base station may correspond to a message requesting thechange of the security key S-K_(eNB) on which the communication betweenmobile station and the secondary base station is based.

In response to the detected or signaled security breach, the master basestation increments, in step 3, a freshness counter maintained for thecommunication between the mobile station and the secondary base station.This freshness counter is for re-initializing the communication betweenthe mobile station and the secondary base station in that it allowsenforcing network access security there between.

Subsequently, the master base station, in step 4, derives on the basisof the incremented freshness counter a new security key S-K_(eNB) forthe communication between the mobile station and the secondary basestation. As discussed in the description above, the derivation of thesecurity key S-K_(eNB) is not only based on the incremented freshnesscounter but also on the security key K_(eNB) for the communicationbetween the mobile station and the master base station which is alsoavailable to both communication partners.

Advantageously, the present embodiment dispenses with the need for themaster base station to re-initialize this communication between themobile station and the master base station, and, hence, with the needfor the master base station to derive a new security key K_(eNB)therefore.

Having derived the new security key S-K_(eNB) for the communicationbetween the mobile station and the secondary base station, the masterbase station generates, in step 5, a re-configuration message includingthe incremented freshness counter for re-initializing the communicationbetween the mobile station and the secondary base station. The masterbase station then transmits same message to the mobile station.

In an implementation of the present embodiment, the re-configurationmessage is an RRC connection reconfiguration message and includesadditional information commonly included in the message. However, in thepresent embodiment, the RRC connection reconfiguration message does notinclude a dedicated preamble for performing a RACH procedure. As notedabove, in the present embodiment, it might be the case that no dedicatedpreambles is or can be assigned.

Subsequently, the master base station transmits, in step 6, the newlyderived security key S-K_(eNB) and the incremented freshness counter tothe secondary base station. From an implementation side, thetransmission of the newly derived security key S-K_(eNB) and theincremented freshness counter may be effected utilizing the X2interface.

The skilled reader can readily appreciate from the discussion above,that the re-configuration message transmitted by the master base stationto the mobile station is ciphered by the master base station using thesecurity key K_(eNB) for communication between each other. In thisrespect, security can be enforced for the transmission of theincremented freshness counter without adding implementation complexity,simply due to the fact that that the present embodiment dispenses withthe need for re-initializing the communication between the mobilestation and the master base station.

Furthermore, from the description above, it becomes apparent that thesteps 5 and 6 may also be carried out by the master base station inreversed order, namely that the re-configuration message including theincremented freshness counter is transmitted (cf. step 5) to the mobilestation after, and not before transmitting (cf. step 6) the newlyderived security key S-K_(eNB) and the incremented freshness counter tothe secondary base station.

In the context of the invention, the reception of this re-configurationmessage by the mobile station may be understood as a trigger for themobile station to perform re-initialization of the communication withthe secondary base station.

Accordingly, the mobile station derives, in step 7, a security keyS-K_(eNB) for the communication with the secondary base station on thebasis of the freshness counter included in the forwardedre-configuration message. This security key S-K_(eNB), which is derivedby the mobile station, is same to the security key S-K_(eNB) derived instep 4 by the master base station and transmitted in step 6 to thesecondary base station.

Consequently, since both, the mobile station and the secondary basestation are provided with the same, derived security key S-K_(eNB), asecure communication link can be established on the basis of thissecurity key S-K_(eNB) between the mobile station and the secondary basestation.

In a variation of step 7, the mobile station first determines whetherthe incremented freshness counter, included in the transmittedre-configuration message, differs from a previous freshness counterincluded in a previously transmitted re-configuration message, and onlyin case of a different, incremented freshness counter, the mobilestation derives the same security key S-K_(eNB).

Subsequently, the mobile station re-initializes, in step 8, thecommunication with the secondary base station on the basis of thederived, same security key (cf. step 8) for the communication betweenthe mobile station and the secondary base station. From theimplementation side, the re-initialization performed by the mobilestation may include re-establishing the PDCP sub-layer, re-establishingthe RLC sub-layer and resetting the MAC layer.

After completion of the re-initialization of the communication with thesecondary base station, the mobile station acknowledges same bytransmitting, in step 9, a re-configuration acknowledge message to thesecondary base station. From an implementation side, the transmission ofthe re-configuration acknowledge message between the base stations instep 13 may be effected utilizing the X2 interface.

In an implementation of the present embodiment, the re-configurationacknowledge message is effected to the secondary base station in form ofa Medium Access Control, MAC, Control Element, CE. Specifically, there-configuration acknowledgement may be included in the transmission ofa MAC control element, by using a specific type of MAC CE for there-configuration message which is indicated by way of a pre-definedLogical Channel ID, LCID in a previous transmission.

Particularly, in this implementation, the re-configuration messageincluding the freshness counter may be transmitted and/or forwarded tothe mobile station (cf. step 5) in form of a MAC packet data unit, PDU.This MAC PDU may include in its MAC header the pre-defined LCIDidentifying the type of MAC CE to be used for the re-configurationacknowledge message in form of the MAC CE. Accordingly, should themobile station in between steps 5 and 9 be required to performtransmissions different from transmitting the re-configurationacknowledge message, in step 9, the mobile station may use MAC CE of adifferent type than that indicated by the pre-defined LCID.

Advantageously, the re-configuration acknowledge message, beingtransmitted by the mobile station to the secondary base station, may beunderstood, in the context of the invention, as a trigger for thesecondary base station to perform re-initialization of the communicationwith the mobile station.

In this respect, the secondary base station re-initializes, in step 10,the communication with the mobile station on the basis of the derived,same security key (cf. steps 4, 6 and 7) for the communication betweenthe mobile station and the secondary base station. From theimplementation side, the re-initialization performed by the mobilestation may include re-establishing the PDCP sub-layer, re-establishingthe RLC sub-layer and resetting the MAC layer.

Consequently, having re-initialized the communication in steps 8 and 10,the mobile station and secondary base station are capable ofestablishing a secure communication link between each other utilizingthe same, derived security key S-K_(eNB).

Advantageously, also in the present embodiment the communication betweenthe mobile station and the secondary base station re-initialized withouthaving to re-initialize the communication between the mobile station andthe master base station. In other words, for the communication betweenthe mobile station and the secondary base station a same secure keyS-K_(eNB) is derived which bases on a incremented, hence different,freshness counter and thereby enables establishing a securecommunication link there between without requiring re-initialization ofthe communication with the master base station, inherently requiringderivation of a corresponding security key K_(eNB).

Further to the implementation of the present embodiment illustrated inFIG. 15, including the re-configuration message in form of a MAC CE, itcan be readily appreciated that due to omission of performing the RACHprocedure, the processing time for establishing the secure communicationlink between the mobile station and the secondary base station decreasesby approximately 5 to 15 ms when compared to the implementation ofsecond example of the more detailed embodiment shown in FIG. 14.

Fourth Example of a More Detailed Embodiment of the Invention

Referring now to FIG. 16 wherein a fourth example of a more detailedembodiment of the invention is shown. This embodiment also illustrates amethod for establishing a secure communication link between a mobilestation and a secondary base station in a mobile communication systememploying dual connectivity. Accordingly, the mobile communicationsystem comprises a master and a secondary base station. The mobilestation is initialized for communicating with the master base stationand the secondary base station.

In contrast to the previous examples of a more detailed embodiment, inthe present embodiment a scenario is assumed where not only a newsecurity key S-K_(eNB) for the communication between the mobile stationand the secondary base station is derived, but also this newly derivedsecurity key S-K_(eNB) is based on a newly derived security key K_(eNB)*for the communication between the mobile station and the master basestation. However, this newly derived security key K_(eNB)* is not usedfor re-initializing the communication between the mobile terminal andthe master base station.

Instead, the new security key K_(eNB)* is only maintained in the masterbase station and the mobile station for the purpose of subsequentlyderiving a new security key S-K_(eNB) for the communication between themobile terminal and the secondary base station.

Advantageously, the present embodiment dispenses with the need for themobile station and the master base station to re-initialize thecommunication between each other. In this respect, also the “old”security key K_(eNB) is maintained in the mobile station and the masterbase station for the communication there between (e.g. for ciphering thecommunication between the mobile station and the master base station).

Supposing that the communication between the mobile station and themaster and the secondary base station is respectively initialized,either the master or the secondary base station perform, in step 1, theoperation of detecting a potential security breach. As discussed above,the potential security breach may be detected as a condition where asequence counter of PDUs is about to wrap-up or where a communicationlink identification is to be reused since initialization of thecommunication between the mobile station and the secondary base station.

In case the secondary base station detects, for example, the conditionwhere a sequence counter of packet data units of an established securecommunication link between the mobile station and the secondary basestation is about to wrap-up or actually wraps-up, same secondary basestation signals, in step 2, the detected security breach to the masterbase station. Since in an alternative case, the master base station mayequally detect a security breach, the signaling of the detected securitybreach is indicated as being optional by way of a dashed line.

Signaling of the detected security breach by the secondary base stationto the master base station may correspond to a message requesting thechange of the security key S-K_(eNB) on which the communication betweenmobile station and the secondary base station is based.

In response to the detected or signaled security breach, the master basestation derives, in step 3, a new security key K_(eNB)* which could beused for re-initializing the communication between the mobile stationand the secondary base station.

However, already at this point of the description, it shall be madeclear that the, in step 3, newly derived security key K_(eNB)* is onlymaintained in the master base station and the mobile station forderiving a new security key S-K_(eNB) for the communication between themobile terminal and the secondary base station (cf. steps 9, 10 and 13).Accordingly, also the “old” security key K_(eNB) is maintained in themobile station and the master base station for communication therebetween (e.g. for ciphering the communication between the mobile stationand the master base station).

Subsequently, the master base station, in step 4, generates are-configuration message including a handover command for thecommunication between the mobile station and the master base station. Aspart of the handover command, information is included which allows (cf.step 5) the mobile station to derive a same, new security key K_(eNB)*which could be used for the communication between it and the master basestation. This re-configuration message including the handover command istransmitted by the master base station to the mobile station.

In an implementation of the present embodiment, the re-configurationmessage is an RRC connection reconfiguration message which includes, ashandover command, mobility control information titled“mobilityControlInfo”. The mobility control information includes, forexample, the Next hop Chaining Counter, NCC, on the basis of which themobile station, upon receipt of the RRC connection configuration messageis capable of deriving a same new security key K_(eNB)* for thecommunication between it and the master base station.

Furthermore, in the implementation of the present embodiment, themobility control information, being the handover command included in theRRC connection reconfiguration message, additionally includes adedicated RACH preamble. For illustrated purposes, the dedicated RACHpreamble is titled RACH preamble-X.

In the context of the invention, the reception of this re-configurationmessage by the mobile station may be understood as a trigger for themobile station to derive the security key K_(eNB)* for communicationbetween the mobile station and the master base station.

In this respect, the mobile station derives, in step 5, a new securitykey K_(eNB)* for the communication with the master base station on thebasis of the NCC included in the handover command transmitted as part ofthe re-configuration message. This security key K_(eNB)*, which isderived by the mobile station, is same to the security key K_(eNB)*derived in step 4 by the master base station and transmitted in step 5to the secondary base station.

Similarly to the above, at this point it shall also be made clear thatthe, in step 5, newly derived security key K_(eNB)* is only maintainedin the mobile station and the master base station for deriving a newsecurity key S-K_(eNB) for the communication between the mobile terminaland the secondary base station (cf. steps 9, 10 and 13). Accordingly,also the “old” security key K_(eNB) is maintained in the mobile stationand the master base station for communication there between (e.g. forciphering the communication between the mobile station and the masterbase station).

Further, it is important to note that the reception of thisre-configuration message by the mobile station does not trigger same toperform on the basis of the newly derived security key K_(eNB)*re-initialization of the communication with the master base station.Accordingly, the mobile station does not take the newly derived securitykey K_(eNB)* as K_(eNB) and does not use this derived security keyK_(eNB)* (or as K_(eNB)) for re-initializing the communication with themaster base station. For illustrative purposes, a hypothetical operationof taking the derived security key K_(eNB)* as K_(eNB) and using it forre-initializing the communication is crossed out, hence indicating thatthis operation is not performed by the mobile station.

Consequently, from the implementation side, the master base station doesnot perform any of the following operations, namely re-establishing thePDCP sub-layer, re-establishing the RLC sub-layer or resetting the MAClayer with respect to the communication between it and the master basestation.

Further to the implementation of the present embodiment where there-configuration message of step 4, being a RRC connectionreconfiguration message, includes dedicated RACH preamble-X, thisdedicated RACH preamble-X is subsequently utilized for performing, instep 6, a RACH procedure between the mobile station and the secondarybase station.

Also in this case, it is important to note that the RACH procedurebetween the mobile station and the master base station only updates thetiming advance for communication there between but cannot be understoodas a trigger for the master base station to perform re-initialization ofthe communication with the mobile station. For illustrative purposes, ahypothetical operation of taking the derived security key K_(eNB)* asK_(eNB) and using it for re-initializing the communication is crossedout, hence indicating that this operation is not performed by the masterbase station.

Consequently, from the implementation side, the master base station doesnot perform any of the following operations, namely re-establishing thePDCP sub-layer, re-establishing the RLC sub-layer or resetting the MAClayer with respect to the communication between it and the master basestation.

Subsequently, the mobile station transmits, in step 7, are-configuration complete message to the master base station. In theimplementation of the present embodiment, in response to there-configuration message, being a RRC connection reconfiguration messagein step 4, the re-configuration complete message is an RRC connectionreconfiguration complete message in step 7.

Advantageously, the re-configuration complete message, being transmittedby the mobile station to the master base station, may be understood, inthe context of the invention, as a trigger for the master base stationto control the mobiles station and the secondary base station to performre-initialization of the communication there between, includingderivation of a same security key S-K_(eNB) on the basis of anincremented freshness counter for establishing a secure communicationlink utilizing same derived security key S-K_(eNB).

In this respect, the master base station increments, in step 8, afreshness counter maintained for the communication between the mobilestation and the secondary base station. This freshness counter is forre-initializing the communication between the mobile station and thesecondary base station in that it allows enforcing network accesssecurity there between.

In this respect, the master base station, in step 9, derives on thebasis of the incremented freshness counter a new security key S-K_(eNB)for the communication between the mobile station and the secondary basestation. As discussed in the description above, the derivation of thesecurity key S-K_(eNB) is not only based on the incremented freshnesscounter but also on a security key K_(eNB)*. Notably, in the presentembodiment, the newly derived security key K_(eNB)* is used by themaster base station in step 8 instead of the “old” security key K_(eNB)which are both maintained in the mobile station and in the master basestation is as discussed above.

Specifically, in steps 3 and 5, the newly derived security key K_(eNB)*is only maintained in the mobile station and the master base station forderiving the new security key S-K_(eNB) for the communication betweenthe mobile terminal and the secondary base station (cf. steps 9, 10 and13) whereas the “old” security key K_(eNB) is maintained in the mobilestation and the master base station for communication there between(e.g. for ciphering the communication between the mobile station and themaster base station).

Having derived the new security key S-K_(eNB) for the communicationbetween the mobile station and the secondary base station, the masterbase station transmits, in step 10, this newly derived security keyS-K_(eNB) and the incremented freshness to the secondary base station.From an implementation side, the transmission of the newly derivedsecurity key S-K_(eNB) and the incremented freshness counter may beeffected utilizing the X2 interface.

Subsequently, the secondary base station generates, in step 11, are-configuration message including the incremented freshness counter forre-initializing the communication between the mobile station and thesecondary base station. The secondary base station then transmits samemessage to the master base station. From an implementation side, thetransmission of the re-configuration message may be effected utilizingthe X2 interface.

In the implementation of the present embodiment, the re-configurationmessage of step 11 is an RRC connection reconfiguration message andincludes additional information commonly included in the message. Inparticular, the RRC connection reconfiguration message additionallyincludes a dedicated Random Access CHannel, RACH, preamble. Forillustrative purposes, the dedicated RACH preamble is titled RACHpreamble-Y.

The re-configuration message, generated by the secondary base station,is then forwarded, in step 12, by the master base station to the mobilestation. Even though the re-configuration message is received from themaster base station, the mobile station can identify from its contentthat it is generated by and, hence, is linked to (i.e. related to) thecommunication with the secondary base station and not to thecommunication with the master base station.

The skilled reader can readily appreciate from the discussion above,that the re-configuration message forwarded by the master base stationto the mobile station is ciphered by the master base station using the“old” security key K_(eNB) for communication between each other. In thisrespect, security can be enforced for the transmission of theincremented freshness counter without adding implementation complexity,simply due to the fact that that the present embodiment dispenses withthe need for re-initializing the communication between the mobilestation and the master base station.

In the context of the invention, the reception of this re-configurationmessage by the mobile station may be understood as a trigger for themobile station to perform re-initialization of the communication withthe secondary base station.

Accordingly, the mobile station derives, in step 13, a security keyS-K_(eNB) for the communication with the secondary base station on thebasis of the freshness counter included in the forwardedre-configuration message. This security key S-K_(eNB), which is derivedby the mobile station, is same to the security key S-K_(eNB) derived instep 4 by the master base station and transmitted in step 5 to thesecondary base station.

As discussed in the description above, the derivation of the securitykey S-K_(eNB) is not only based on the incremented freshness counter butalso on a security key K_(eNB)*. Notably, in the present embodiment, thenewly derived security key K_(eNB)* is used by the mobile station instep 13 instead of the “old” security key K_(eNB) which are bothmaintained in the mobile station and in the master base station is asdiscussed above.

Consequently, since both, the mobile station and the secondary basestation are provided with the same, derived security key S-K_(eNB), asecure communication link can be established on the basis of thissecurity key S-K_(eNB) between the mobile station and the secondary basestation.

In a variation of step 13, the mobile station first determines whetherthe incremented freshness counter, included in the transmittedre-configuration message, differs from a previous freshness counterincluded in a previously transmitted re-configuration message, and onlyin case of a different, incremented freshness counter, the mobilestation derives the same security key S-K_(eNB).

Subsequently, the mobile station re-initializes, in step 14, thecommunication with the secondary base station on the basis of thederived, same security key (cf. step 13) for the communication betweenthe mobile station and the secondary base station. From theimplementation side, the re-initialization performed by the mobilestation may include re-establishing the PDCP sub-layer, re-establishingthe RLC sub-layer and resetting the MAC layer.

Further to the implementation of the present embodiment, there-configuration message, being a RRC connection reconfigurationmessage, includes dedicated RACH preamble-y, which is subsequentlyutilized for performing, in step 15, a RACH procedure between the mobilestation and the secondary base station.

Advantageously, the RACH procedure between the mobile station and thesecondary base station not only updates the timing advance forcommunication there between but also may be understood, in the contextof the invention, as a trigger for the secondary base station to performre-initialization of the communication with the mobile station.

In this respect, the secondary base station re-initializes, in step 16,the communication with the mobile station on the basis of the derived,same security key (cf. step 4 and 5) for the communication between themobile station and the secondary base station. From the implementationside, the re-initialization performed by the mobile station may includere-establishing the PDCP sub-layer, re-establishing the RLC sub-layerand resetting the MAC layer.

Subsequently, the mobile station transmits, in step 17, are-configuration complete message to the master base station which, instep 18, is forwarded by the master base station to the secondary basestation. Particularly, in the present embodiment, the re-configurationcomplete message is transmitted by the mobile station after the mobilestation and the secondary base station have both re-initialized thecommunication there between.

This may be achieved, for example, by the mobile station deferring thetransmission of the re-configuration complete message to the master basestation by a predetermined time interval. Alternatively, the mobilestation may also assume that the secondary base station re-initializesin step 16 its connection faster than the transmission and theforwarding of the re-configuration complete message in steps 17 and 18and may immediately transmit same message after completion of the RACHprocedure in step 15.

The skilled reader can readily appreciate from the discussion above,that the re-configuration complete message, forwarded by the mobilestation to the master base station, is ciphered by the mobile stationusing the “old” security key K_(eNB) for communication between eachother. In this respect, security can be enforced also for thetransmission of this re-configuration complete message. From animplementation side, the transmission of the re-configuration completemessage between the base stations in step 13 may be effected via the X2interface.

Even further to the implementation of the present embodiment, inresponse to the re-configuration message, being a RRC connectionreconfiguration message, the re-configuration complete message is an RRCconnection reconfiguration complete message.

Consequently, having re-initialized the communication in steps 14 and16, the mobile station and secondary base station are capable ofestablishing a secure communication link between each other utilizingthe same, derived security key S-K_(eNB).

Advantageously, also in the present embodiment the communication betweenthe mobile station and the secondary base station re-initialized withouthaving to re-initialize the communication between the mobile station andthe master base station. In other words, for the communication betweenthe mobile station and the secondary base station a same secure keyS-K_(eNB) is derived which bases on a incremented, hence different,freshness counter and thereby enables establishing a securecommunication link there between without requiring re-initialization ofthe communication with the master base station.

In summary, the above described fourth example offers with respect to aconventional Intra-cell handover the following benefits. Firstly, thesecure communication links established (i.e. radio bearers) establishedin the macro cell group, MCG, will not be interrupted un-necessarily.Secondly, the time for establishing a secure communication link in caseof a detected potential security breach including derivation (i.e.refreshing) new security key S-KeNB will be faster because the securekey S-KeNB derivation will no longer depend on the completion ofintra-cell handover. Finally, a special UE behavior/implementation isnot required: in the state-of-the-art a UE/network releases thesecondary cell group, SCG, during MeNB-SeNB handover which also includesan intra-cell handover. To be able to use intra-cell handover to enforcenetwork access security, the SCG release is not be performed to refreshthe S-KeNB which in itself is a new behavior to the commonly knownintra-cell handover.

In FIG. 16, the intra-cell handover for changing the K_(eNB) of steps4-7 and the procedure of deriving S-K_(eNB) in steps 8-18 are twoseparate procedures. According to a variation of the above describedfourth example of a more detailed embodiment, the two procedures arecombined resulting in the UE deriving the K_(eNB) and also the S-K_(eNB)from the new K_(eNB) at a same time, namely upon receiving intra-cellhandover command termed mobility control info and included in the RRCconnection reconfiguration message in step 4.

In this variation it must be clarified based on what input parametersthe UE decides to (re)derive the S-K_(eNB). In this respect, it shouldbe noted that the Next hop Chaining Counter, NCC, alone is not besufficient. Consequently, it is proposed to additionally provide anincremented freshness parameter. Upon detecting an incremented freshnessparameter, the UE then (re)derive the S-K_(eNB) from the new K_(eNB) andincremented freshness parameter. In another simplified version of thisvariation, the UE refreshes/re-derives the S-K_(eNB) whenever afreshness parameter is sent/indicated by the network without the UEchecking if the received freshness parameter is incremented with respectto the previously received freshness counter (i.e. the last freshnessparameter stored by the UE, and was used for a successful S-K_(eNB)derivation). In summary, also here the S-K_(eNB) derivation is alwaysbased on most recently derived K_(eNB) and the most recently receivedfreshness parameter.

Hardware and Software Implementation of the Invention

Another embodiment of the invention relates to the implementation of theabove described various embodiments using hardware and software, orhardware only. In this connection the invention provides an userequipment (mobile terminal) and a master and secondary eNodeB (basestation). The user equipment and base station are adapted to perform themethods described herein.

It is further recognized that the various embodiments of the inventionmay be implemented or performed using computing devices (processors). Acomputing device or processor may for example be general purposeprocessors, digital signal processors (DSP), application specificintegrated circuits (ASIC), field programmable gate arrays (FPGA) orother programmable logic devices, etc. In addition, radio transmitterand radio receiver and other necessary hardware may be provided in theapparatuses (UE, MeNB, SeNB). The various embodiments of the inventionmay also be performed or embodied by a combination of these devices.

Further, the various embodiments of the invention may also beimplemented by means of software modules, which are executed by aprocessor or directly in hardware. Also a combination of softwaremodules and a hardware implementation may be possible. The softwaremodules may be stored on any kind of computer readable storage media,for example RAM, EPROM, EEPROM, flash memory, registers, hard disks,CD-ROM, DVD, etc.

It should be further noted that the individual features of the differentembodiments of the invention may individually or in arbitrarycombination be subject matter to another invention.

It would be appreciated by a person skilled in the art that numerousvariations and/or modifications may be made to the present invention asshown in the specific embodiments without departing from the spirit orscope of the invention as broadly described. The present embodimentsare, therefore, to be considered in all respects to be illustrative andnot restrictive.

1. A method for establishing a secure communication link between amobile station and a secondary base station in a mobile communicationsystem comprising the mobile station, a master and the secondary basestation, the mobile station being initialized for communication with themaster and the secondary base station, detecting, by the master or bythe secondary base station, a potential security breach including: acondition where a sequence counter of packet data units of anestablished secure communication link between the mobile station and thesecondary base station is to wrap-up since initialization of thecommunication between the mobile station and the secondary base station,and a condition where a communication link identification is to bereused for establishing the secure communication link with the secondarybase station since initialization of the communication between themobile station and the secondary base station; and, in case thepotential security breach is detected by the secondary base station,signaling the detected security breach to the master base station;incrementing, by the master base station, in response to the detected orsignaled potential security breach, a freshness counter forre-initializing the communication between the mobile station and thesecondary base station; and re-initializing, by the mobile station andby the secondary base station, the communication there between, there-initializing step being performed under the control of the masterbase station and further includes deriving a same security key based onsaid incremented freshness counter, and establishing the securecommunication link utilizing the same, derived security key.
 2. Themethod according to claim 1, wherein the re-initialization step isperformed by the mobile station and by the secondary base stationwithout re-initializing the communication between the mobile station andthe master base station.
 3. The method according to claim 1, wherein thesecondary base station performs the step of detecting the conditionwhere the sequence counter is to wrap-up, and wherein either the masteror the secondary base station performs the step of detecting thecondition where a communication link identification is to be reused. 4.The method according to claim 1, wherein the step of re-initializing thecommunication between the mobile station and the secondary base stationfurther includes: deriving, by the master base station, a security keybased on the incremented freshness counter for the communication betweenthe mobile station and the secondary base station, and transmitting saidderived security key and the incremented freshness counter to thesecondary base station; generating, by the secondary base station, are-configuration message for re-initializing the communication betweenthe mobile station and the secondary base station including theincremented freshness counter and transmitting the re-configurationmessage to the master base station; forwarding, by the master basestation, the re-configuration message including the incrementedfreshness counter to the mobile station; and deriving, by the mobilestation, a same security key based on the freshness counter included inthe forwarded re-configuration message for establishing on the basis ofthe same, derived security key the secure communication link between themobile station and the secondary base station.
 5. The method accordingto claim 1, wherein the step of re-initializing the communicationbetween the mobile station and the secondary base station furtherincludes: generating and transmitting, by the master base station, are-configuration message for re-initializing the communication betweenthe mobile station and the secondary base station including theincremented freshness counter to the mobile station; deriving, by themaster base station, a security key based on the incremented freshnesscounter for the communication between the mobile station and thesecondary base station, and transmitting, by the master base station,said derived security key to the secondary base station; deriving, bythe mobile station, a same security key based on the freshness counterincluded in the transmitted re-configuration message for establishing onthe basis of the same, derived security key the secure communicationlink between the mobile station and the secondary base station; andwherein the step of deriving and transmitting said derived security keyto the secondary base station is performed, by the master base station,before or after the step of transmitting the re-configuration messageincluding the incremented freshness counter to the mobile station isperformed by the master base station.
 6. The method according to claim5, wherein the master base station additionally transmits theincremented freshness counter to the secondary base station.
 7. Themethod according to claim 4, wherein the step of deriving, by the mobilestation, a same security key based on the incremented freshness counterincluded in the transmitted re-configuration message, further includes:determining, by the mobile station, whether the incremented freshnesscounter, included in the transmitted re-configuration message, differsfrom a previous freshness counter included in a previously transmittedre-configuration message, and only in case of a different, incrementedfreshness counter, performing the step of deriving the same security keyby the mobile station.
 8. The method according to claim 4, furthercomprising the step of transmitting, by the mobile station, are-configuration complete message to the master or to the secondary basestation; and, in case the re-configuration complete message istransmitted by the mobile station to the master base station, the stepof forwarding, by the master base station, said re-configurationcomplete message to the secondary base station.
 9. The method accordingto claim 8, wherein the step of transmitting, and forwarding thereconfiguration complete message is performed after the step ofre-initializing, by the mobile station and the secondary base stationthe communication there between.
 10. The method according to claim 4,wherein the re-configuration message, generated by the master or thesecondary base station, further includes a dedicated preamble orindicates a dedicated preamble from among a plurality of pre-configuredpreambles for performing a Random Access CHannel, RACH, procedurebetween the mobile station and the secondary base station; and themethod further includes the step of: performing between the mobilestation and the secondary base station the RACH procedure utilizing thededicated preamble included in the re-configuration message or utilizingthe indicated dedicated preamble from the plurality of pre-configuredpreambles, respectively.
 11. The method according to claim 10, whereinthe step of performing the RACH procedure is performed between themobile station and the secondary base station after the step ofre-initializing, by the mobile station, the communication with thesecondary base station, and before the step of re-initializing, bysecondary base station, the communication with the mobile station. 12.The method according to claim 10, wherein the plurality ofpre-configured preambles, from among which the re-configuration messageindicates the dedicated preamble to be used for performing the RACHprocedure, are pre-configured using an X2 interface between the masterand the secondary base station.
 13. The method according to claim 4,wherein the step of transmitting and/or forwarding the re-configurationmessage is performed by signaling a corresponding Radio ResourceControl, RRC, connection reconfiguration message, and, in case of are-configuration complete message, the step of transmitting and/orforwarding the re-configuration complete message is performed bysignaling a corresponding RRC connection reconfiguration completemessage.
 14. The method according to claim 4, wherein the method furtherincludes the step of: transmitting, by the mobile station, in responseto the re-configuration message including a freshness counter, are-configuration acknowledge message to the secondary base station inform of a Medium Access Control, MAC, Control Element, CE.
 15. Themethod according to claim 14, wherein the step of transmitting there-configuration acknowledge message is performed before the step ofre-initializing, by the secondary base station, the communication withthe mobile station.
 16. The method according to claim 14, wherein there-configuration message including the freshness counter is transmittedand/or forwarded to the mobile station in form of MAC packet data unit,PDU, including in its MAC header a pre-defined Logical Channel ID, LCID,and wherein the LCID identifies the type of the MAC CE for there-configuration acknowledge message.
 17. A mobile communication systemfor establishing a secure communication link between a mobile stationand a secondary base station, the mobile communication system comprisingthe mobile station, a master and the secondary base station, the mobilestation being initialized for communication with the master and thesecondary base station, wherein: the master and/or the secondary basestation are configured to detect a potential security breach including:a condition where a sequence counter of packet data units of anestablished secure communication link between the mobile station and thesecondary base station is to wrap-up since initialization of thecommunication between the mobile station and the secondary base station,and a condition where a communication link identification is to bereused for establishing the secure communication link with the secondarybase station since initialization of the communication between themobile station and the secondary base station; the secondary basestation is configured to, in case of detecting the potential securitybreach, signal the detected security breach to the master base station;the master base station is configured to increment in response to thedetected or signaled potential security breach, a freshness counter forre-initializing the communication between the mobile station and thesecondary base station; and the mobile station and the secondary basestation are further configured to re-initialize the communication therebetween, the re-initialization being performed under the control of themaster base station and includes deriving a same security key based onsaid incremented freshness counter, and establishing the securecommunication link utilizing the same, derived security key.
 18. Themobile communication system according to claim 17, wherein the mobilestation and the secondary base station are configured to re-initialize,under the control of the master base station, the communication therebetween without re-initializing the communication between the mobilestation and the master base station.
 19. The mobile communication systemaccording to claim 17, wherein the secondary base station is configuredto detect the condition where the sequence counter is to wrap-up, andwherein either the master or the secondary base station are configuredto detect the condition where a communication link identification is tobe reused.
 20. The mobile communication system according to claim 17,wherein: the master base station is configured to derive a security keybased on the incremented freshness counter for the communication betweenthe mobile station and the secondary base station, and to transmit saidderived security key and the incremented freshness counter to thesecondary base station; the secondary base station is configured togenerate a re-configuration message for the communication between themobile station and the secondary base station including the incrementedfreshness counter and to transmit the re-configuration message to themaster base station; the master base station is configured to forwardthe re-configuration message including the incremented freshness counterto the mobile station; and the mobile station is configured to derive asame security key based on the freshness counter included in theforwarded re-configuration message for establishing on the basis of thesame, derived security key the secure communication link between themobile station and the secondary base station. 21.-33. (canceled)